Other mistake: Marlowe fires several shots at Art Huck and Lash Canino but if you look at the chamber when he takes the gun out from under the dashboard, there are no bullets in it. (01:41:45)
Continuity mistake: When the state trooper examines the letter, the is no stamp. However, when Charlie Chan (Sydney Toler) examines the letter, there is a stamp present. (00:43:10)
Continuity mistake: The plot is based on obtaining 3 music boxes, identical in appearance each playing a slight variation of the same tune. Throughout the movie the variations are highlighted as a message is revealed by listening to all 3 variations. Yet at the beginning of the movie when the boxes are all sold at the same auction, when each box is presented, the tune had no variations.
Continuity mistake: The postman hangs his bag to the hook on the wall. By the noise, he knows a split-second in advance that an explosion is about to happen, and ducks. The actor ducks out of view; we are not supposed to notice that once he disappears, film editing inserts the part when the background partially detonates. We however do, because the bag in the foreground changes position in the space of a single frame. (00:05:25)