Missing Episode: The film was envisioned as a collection of several short stories, with two more segments filmed but ultimately left out of the film to keep the running time down. One of them is included in some home video releases.
The original idea had four stories involved, two of which (one in which Watson solves a mystery on a cruise and one in which Holmes solves a murder in an upside-down room) were edited out. The latter featured Inspector Lestrade.
Wilder originally planned on casting Peter O'Toole as Holmes and Peter Sellers as Watson.
George Sanders was supposed to play Mycroft, but dropped out at the last minute and was replaced by Christopher Lee.
Word of Dante: The movie takes Holmes' older, smarter brother Mycroft and suggests that he was the head of the Secret Service, and that his frequent haunt, the Diogenes Club, was a front. In canon, neither are other than what they appear to be (a Brilliant, but Lazy civil servant and a club of reclusive eccentrics specifically), but this interpretation has become a popular one.
That grave digger! It's the same one from Hamlet—the one with Laurence Olivier, no less. And he looks just like Alfred P. Doolittle.