diff --git a/synapse/util/wheel_timer.py b/synapse/util/wheel_timer.py
index ab5d5a27de77..f36a2c9147c6 100644
--- a/synapse/util/wheel_timer.py
+++ b/synapse/util/wheel_timer.py
@@ -11,8 +11,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import logging
 from typing import Generic, Hashable, List, Set, TypeVar
 
+logger = logging.getLogger(__name__)
+
 T = TypeVar("T", bound=Hashable)
 
 
@@ -51,17 +54,27 @@ def insert(self, now: int, obj: T, then: int) -> None:
             then: When to return the object strictly after.
         """
         then_key = int(then / self.bucket_size) + 1
+        now_key = int(now / self.bucket_size)
 
         if self.entries:
             min_key = self.entries[0].end_key
             max_key = self.entries[-1].end_key
 
+            if min_key < now_key - 10:
+                # If we have ten buckets that are due and still nothing has
+                # called `fetch()` then we likely have a bug that is causing a
+                # memory leak.
+                logger.warning(
+                    "Inserting into a wheel timer that hasn't been read from recently. Item: %s",
+                    obj,
+                )
+
             if then_key <= max_key:
                 # The max here is to protect against inserts for times in the past
                 self.entries[max(min_key, then_key) - min_key].queue.add(obj)
                 return
 
-        next_key = int(now / self.bucket_size) + 1
+        next_key = now_key + 1
         if self.entries:
             last_key = self.entries[-1].end_key
         else: