diff --git a/changelog.d/14922.misc b/changelog.d/14922.misc
new file mode 100644
index 0000000000..2cc3614dfd
--- /dev/null
+++ b/changelog.d/14922.misc
@@ -0,0 +1 @@
+Use `StrCollection` to avoid potential bugs with `Collection[str]`.
diff --git a/synapse/handlers/account_data.py b/synapse/handlers/account_data.py
index 834006356a..d500b21809 100644
--- a/synapse/handlers/account_data.py
+++ b/synapse/handlers/account_data.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 import logging
 import random
-from typing import TYPE_CHECKING, Awaitable, Callable, Collection, List, Optional, Tuple
+from typing import TYPE_CHECKING, Awaitable, Callable, List, Optional, Tuple
 
 from synapse.api.constants import AccountDataTypes
 from synapse.replication.http.account_data import (
@@ -26,7 +26,7 @@ from synapse.replication.http.account_data import (
     ReplicationRemoveUserAccountDataRestServlet,
 )
 from synapse.streams import EventSource
-from synapse.types import JsonDict, StreamKeyType, UserID
+from synapse.types import JsonDict, StrCollection, StreamKeyType, UserID
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -322,7 +322,7 @@ class AccountDataEventSource(EventSource[int, JsonDict]):
         user: UserID,
         from_key: int,
         limit: int,
-        room_ids: Collection[str],
+        room_ids: StrCollection,
         is_guest: bool,
         explicit_room_id: Optional[str] = None,
     ) -> Tuple[List[JsonDict], int]:
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 58180ae2fa..5c06073901 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -18,7 +18,6 @@ from http import HTTPStatus
 from typing import (
     TYPE_CHECKING,
     Any,
-    Collection,
     Dict,
     Iterable,
     List,
@@ -45,6 +44,7 @@ from synapse.metrics.background_process_metrics import (
 )
 from synapse.types import (
     JsonDict,
+    StrCollection,
     StreamKeyType,
     StreamToken,
     UserID,
@@ -146,7 +146,7 @@ class DeviceWorkerHandler:
 
     @cancellable
     async def get_device_changes_in_shared_rooms(
-        self, user_id: str, room_ids: Collection[str], from_token: StreamToken
+        self, user_id: str, room_ids: StrCollection, from_token: StreamToken
     ) -> Set[str]:
         """Get the set of users whose devices have changed who share a room with
         the given user.
@@ -551,7 +551,7 @@ class DeviceHandler(DeviceWorkerHandler):
     @trace
     @measure_func("notify_device_update")
     async def notify_device_update(
-        self, user_id: str, device_ids: Collection[str]
+        self, user_id: str, device_ids: StrCollection
     ) -> None:
         """Notify that a user's device(s) has changed. Pokes the notifier, and
         remote servers if the user is local.
diff --git a/synapse/handlers/event_auth.py b/synapse/handlers/event_auth.py
index f91dbbecb7..a23a8ce2a1 100644
--- a/synapse/handlers/event_auth.py
+++ b/synapse/handlers/event_auth.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import TYPE_CHECKING, Collection, List, Mapping, Optional, Union
+from typing import TYPE_CHECKING, List, Mapping, Optional, Union
 
 from synapse import event_auth
 from synapse.api.constants import (
@@ -29,7 +29,7 @@ from synapse.event_auth import (
 )
 from synapse.events import EventBase
 from synapse.events.builder import EventBuilder
-from synapse.types import StateMap, get_domain_from_id
+from synapse.types import StateMap, StrCollection, get_domain_from_id
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -290,7 +290,7 @@ class EventAuthHandler:
 
     async def get_rooms_that_allow_join(
         self, state_ids: StateMap[str]
-    ) -> Collection[str]:
+    ) -> StrCollection:
         """
         Generate a list of rooms in which membership allows access to a room.
 
@@ -331,7 +331,7 @@ class EventAuthHandler:
 
         return result
 
-    async def is_user_in_rooms(self, room_ids: Collection[str], user_id: str) -> bool:
+    async def is_user_in_rooms(self, room_ids: StrCollection, user_id: str) -> bool:
         """
         Check whether a user is a member of any of the provided rooms.
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 233f8c113d..dc1cbf5c3d 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -20,17 +20,7 @@ import itertools
 import logging
 from enum import Enum
 from http import HTTPStatus
-from typing import (
-    TYPE_CHECKING,
-    Collection,
-    Dict,
-    Iterable,
-    List,
-    Optional,
-    Set,
-    Tuple,
-    Union,
-)
+from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Set, Tuple, Union
 
 import attr
 from prometheus_client import Histogram
@@ -70,7 +60,7 @@ from synapse.replication.http.federation import (
 )
 from synapse.storage.databases.main.events import PartialStateConflictError
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
-from synapse.types import JsonDict, get_domain_from_id
+from synapse.types import JsonDict, StrCollection, get_domain_from_id
 from synapse.types.state import StateFilter
 from synapse.util.async_helpers import Linearizer
 from synapse.util.retryutils import NotRetryingDestination
@@ -179,7 +169,7 @@ class FederationHandler:
         # A dictionary mapping room IDs to (initial destination, other destinations)
         # tuples.
         self._partial_state_syncs_maybe_needing_restart: Dict[
-            str, Tuple[Optional[str], Collection[str]]
+            str, Tuple[Optional[str], StrCollection]
         ] = {}
         # A lock guarding the partial state flag for rooms.
         # When the lock is held for a given room, no other concurrent code may
@@ -437,7 +427,7 @@ class FederationHandler:
             )
         )
 
-        async def try_backfill(domains: Collection[str]) -> bool:
+        async def try_backfill(domains: StrCollection) -> bool:
             # TODO: Should we try multiple of these at a time?
 
             # Number of contacted remote homeservers that have denied our backfill
@@ -1730,7 +1720,7 @@ class FederationHandler:
     def _start_partial_state_room_sync(
         self,
         initial_destination: Optional[str],
-        other_destinations: Collection[str],
+        other_destinations: StrCollection,
         room_id: str,
     ) -> None:
         """Starts the background process to resync the state of a partial state room,
@@ -1812,7 +1802,7 @@ class FederationHandler:
     async def _sync_partial_state_room(
         self,
         initial_destination: Optional[str],
-        other_destinations: Collection[str],
+        other_destinations: StrCollection,
         room_id: str,
     ) -> None:
         """Background process to resync the state of a partial-state room
@@ -1949,9 +1939,9 @@ class FederationHandler:
 
 def _prioritise_destinations_for_partial_state_resync(
     initial_destination: Optional[str],
-    other_destinations: Collection[str],
+    other_destinations: StrCollection,
     room_id: str,
-) -> Collection[str]:
+) -> StrCollection:
     """Work out the order in which we should ask servers to resync events.
 
     If an `initial_destination` is given, it takes top priority. Otherwise
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 904a721483..e037acbca2 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -80,6 +80,7 @@ from synapse.types import (
     PersistedEventPosition,
     RoomStreamToken,
     StateMap,
+    StrCollection,
     UserID,
     get_domain_from_id,
 )
@@ -615,7 +616,7 @@ class FederationEventHandler:
 
     @trace
     async def backfill(
-        self, dest: str, room_id: str, limit: int, extremities: Collection[str]
+        self, dest: str, room_id: str, limit: int, extremities: StrCollection
     ) -> None:
         """Trigger a backfill request to `dest` for the given `room_id`
 
@@ -1565,7 +1566,7 @@ class FederationEventHandler:
     @trace
     @tag_args
     async def _get_events_and_persist(
-        self, destination: str, room_id: str, event_ids: Collection[str]
+        self, destination: str, room_id: str, event_ids: StrCollection
     ) -> None:
         """Fetch the given events from a server, and persist them as outliers.
 
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index 8c8ff18a1a..1fe6567185 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import TYPE_CHECKING, Collection, Dict, List, Optional, Set
+from typing import TYPE_CHECKING, Dict, List, Optional, Set
 
 import attr
 
@@ -28,7 +28,7 @@ from synapse.logging.opentracing import trace
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.rest.admin._base import assert_user_is_admin
 from synapse.streams.config import PaginationConfig
-from synapse.types import JsonDict, Requester, StreamKeyType
+from synapse.types import JsonDict, Requester, StrCollection, StreamKeyType
 from synapse.types.state import StateFilter
 from synapse.util.async_helpers import ReadWriteLock
 from synapse.util.stringutils import random_string
@@ -391,7 +391,7 @@ class PaginationHandler:
         """
         return self._delete_by_id.get(delete_id)
 
-    def get_delete_ids_by_room(self, room_id: str) -> Optional[Collection[str]]:
+    def get_delete_ids_by_room(self, room_id: str) -> Optional[StrCollection]:
         """Get all active delete ids by room
 
         Args:
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 572c7b4db3..60a6d9cf3c 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -20,16 +20,7 @@ import random
 import string
 from collections import OrderedDict
 from http import HTTPStatus
-from typing import (
-    TYPE_CHECKING,
-    Any,
-    Awaitable,
-    Collection,
-    Dict,
-    List,
-    Optional,
-    Tuple,
-)
+from typing import TYPE_CHECKING, Any, Awaitable, Dict, List, Optional, Tuple
 
 import attr
 from typing_extensions import TypedDict
@@ -72,6 +63,7 @@ from synapse.types import (
     RoomID,
     RoomStreamToken,
     StateMap,
+    StrCollection,
     StreamKeyType,
     StreamToken,
     UserID,
@@ -1644,7 +1636,7 @@ class RoomEventSource(EventSource[RoomStreamToken, EventBase]):
         user: UserID,
         from_key: RoomStreamToken,
         limit: int,
-        room_ids: Collection[str],
+        room_ids: StrCollection,
         is_guest: bool,
         explicit_room_id: Optional[str] = None,
     ) -> Tuple[List[EventBase], RoomStreamToken]:
diff --git a/synapse/handlers/room_summary.py b/synapse/handlers/room_summary.py
index c6b869c6f4..4472019fbc 100644
--- a/synapse/handlers/room_summary.py
+++ b/synapse/handlers/room_summary.py
@@ -36,7 +36,7 @@ from synapse.api.errors import (
 )
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.events import EventBase
-from synapse.types import JsonDict, Requester
+from synapse.types import JsonDict, Requester, StrCollection
 from synapse.util.caches.response_cache import ResponseCache
 
 if TYPE_CHECKING:
@@ -870,7 +870,7 @@ class _RoomQueueEntry:
     # The room ID of this entry.
     room_id: str
     # The server to query if the room is not known locally.
-    via: Sequence[str]
+    via: StrCollection
     # The minimum number of hops necessary to get to this room (compared to the
     # originally requested room).
     depth: int = 0
diff --git a/synapse/handlers/search.py b/synapse/handlers/search.py
index 40f4635c4e..9bbf83047d 100644
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@@ -14,7 +14,7 @@
 
 import itertools
 import logging
-from typing import TYPE_CHECKING, Collection, Dict, Iterable, List, Optional, Set, Tuple
+from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Set, Tuple
 
 import attr
 from unpaddedbase64 import decode_base64, encode_base64
@@ -23,7 +23,7 @@ from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import NotFoundError, SynapseError
 from synapse.api.filtering import Filter
 from synapse.events import EventBase
-from synapse.types import JsonDict, StreamKeyType, UserID
+from synapse.types import JsonDict, StrCollection, StreamKeyType, UserID
 from synapse.types.state import StateFilter
 from synapse.visibility import filter_events_for_client
 
@@ -418,7 +418,7 @@ class SearchHandler:
     async def _search_by_rank(
         self,
         user: UserID,
-        room_ids: Collection[str],
+        room_ids: StrCollection,
         search_term: str,
         keys: Iterable[str],
         search_filter: Filter,
@@ -491,7 +491,7 @@ class SearchHandler:
     async def _search_by_recent(
         self,
         user: UserID,
-        room_ids: Collection[str],
+        room_ids: StrCollection,
         search_term: str,
         keys: Iterable[str],
         search_filter: Filter,
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 44e70fc4b8..4a27c0f051 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -20,7 +20,6 @@ from typing import (
     Any,
     Awaitable,
     Callable,
-    Collection,
     Dict,
     Iterable,
     List,
@@ -47,6 +46,7 @@ from synapse.http.server import respond_with_html, respond_with_redirect
 from synapse.http.site import SynapseRequest
 from synapse.types import (
     JsonDict,
+    StrCollection,
     UserID,
     contains_invalid_mxid_characters,
     create_requester,
@@ -141,7 +141,8 @@ class UserAttributes:
     confirm_localpart: bool = False
     display_name: Optional[str] = None
     picture: Optional[str] = None
-    emails: Collection[str] = attr.Factory(list)
+    # mypy thinks these are incompatible for some reason.
+    emails: StrCollection = attr.Factory(list)  # type: ignore[assignment]
 
 
 @attr.s(slots=True, auto_attribs=True)
@@ -159,7 +160,7 @@ class UsernameMappingSession:
 
     # attributes returned by the ID mapper
     display_name: Optional[str]
-    emails: Collection[str]
+    emails: StrCollection
 
     # An optional dictionary of extra attributes to be provided to the client in the
     # login response.
@@ -174,7 +175,7 @@ class UsernameMappingSession:
     # choices made by the user
     chosen_localpart: Optional[str] = None
     use_display_name: bool = True
-    emails_to_use: Collection[str] = ()
+    emails_to_use: StrCollection = ()
     terms_accepted_version: Optional[str] = None
 
 
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index ee11764567..9e9601d423 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -17,7 +17,6 @@ from typing import (
     TYPE_CHECKING,
     AbstractSet,
     Any,
-    Collection,
     Dict,
     FrozenSet,
     List,
@@ -62,6 +61,7 @@ from synapse.types import (
     Requester,
     RoomStreamToken,
     StateMap,
+    StrCollection,
     StreamKeyType,
     StreamToken,
     UserID,
@@ -1179,7 +1179,7 @@ class SyncHandler:
     async def _find_missing_partial_state_memberships(
         self,
         room_id: str,
-        members_to_fetch: Collection[str],
+        members_to_fetch: StrCollection,
         events_with_membership_auth: Mapping[str, EventBase],
         found_state_ids: StateMap[str],
     ) -> StateMap[str]:
diff --git a/synapse/rest/client/push_rule.py b/synapse/rest/client/push_rule.py
index 8191b4e32c..ad5c10c99d 100644
--- a/synapse/rest/client/push_rule.py
+++ b/synapse/rest/client/push_rule.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import TYPE_CHECKING, List, Sequence, Tuple, Union
+from typing import TYPE_CHECKING, List, Tuple, Union
 
 from synapse.api.errors import (
     NotFoundError,
@@ -169,7 +169,7 @@ class PushRuleRestServlet(RestServlet):
             raise UnrecognizedRequestError()
 
 
-def _rule_spec_from_path(path: Sequence[str]) -> RuleSpec:
+def _rule_spec_from_path(path: List[str]) -> RuleSpec:
     """Turn a sequence of path components into a rule spec
 
     Args: