Continuity mistake: The postman walks into the tavern as the bell tolls. When he sits at the table with the others, everyone changed position at the cut. The bearded man had his hand on his glass, and now it's gone, and the priest's hand was not on the table, but then it is. (00:02:15)
Revealing mistake: After Holmes reveals that Mr. Potts is Ransom, the detective and the murderer have a conversation. After Holmes says 'No, not necessarily', the shot changes. In the next shot, you can see that the man standing in front of Basil Rathbone is not Gerald Hamer anymore. This new man has much less hair than Hamer and has a broader forehead. (01:10:39)
