Continuity mistake: When Watson is conned by the prostitute who says she knows Mary Kelly, she lures him into a closed alley. After Watson has called the police and subdued the attacker, 2 constables arrive and arrest Watson, but as they all leave, a third constable comes from behind the camera. Either he magically appeared, or he walked through the wall behind.
Murder by Decree (1979)
1 continuity mistake
Directed by: Bob Clark
Starring: Christopher Plummer, James Mason, David Hemmings, Susan Clark
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.