Continuity mistake: After Mrs. Monteith hands MacGregor's letter to Lestrade, she walks behind Watson and begins to walk behind Holmes when the shot changes. In the next shot, which focuses on Holmes opening Lestrade's envelope, Mrs. Monteith has disappeared.
Revealing mistake: The wide shot of the exterior of Drearcliffe House is a recycling of the wide shot of the church where Meade and his Nazi colleagues were hiding in "Sherlock Holmes and the Voice of Terror." It can't be the same place, as the mansion is located in Scotland and the church in England. (00:01:21)
