Dressed to Kill (1946)

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.

kaevanoff

Holmes: However, it's a mistake to accept something that's true merely because it's obvious.