Revealing mistake: In the first scene on Ginger's sister's boat that's being refitted, one of the seagulls (yes they used real seagulls) flies over the stern end right into the cyclorama and takes a nose dive to the stage floor.
Revealing mistake: When Cabal and Passworthy look upon the distant searchlights from their doorstep, it's plain that the "beams" of light are really just solid dowel rods being swung about.
Revealing mistake: During the fight in the prison, you can see that the doors are really cheap. A policeman is pushed on one of them and it opens. Not really efficient to retain prisoneers. (00:26:40)
Revealing mistake: During a fight scene in which a bad guy flips James Cagney's character over his shoulder and slams him against a wall, you can see that it isn't Cagney being flipped. The stunt double's face is briefly visible.