Revealing mistake: As Holmes and Watson are riding home from Watson's stay in prison, in the background double traffic lines are visible as are concrete traffic barriers. These are a modern invention and out of place.
Revealing mistake: At the end, when the inspector is stabbed by one of the murderers, he falls back into a sturdy looking fence that breaks apart. As he is lying there, you can see the pre-cuts on the wood so that the fence would break cleanly in a specific spot.
