Other mistake: When Farley Granger sneaks out of his flat to go to Robert Walker's house, there is an EXIT sign over the door. Maybe in a theater or cinema, but in a private apartment?
Suggested correction: 1) This only appears in the preview version of the film, not the final version. 2) Why would an apartment building not have an exit sign? Can you cite a reference for this? I would think someone making a contemporary film would notice if it was such an obvious error you think it is.
Other mistake: When Bruno returns to Metcalf to plant the lighter at the fairground, the passenger to his right in the club car asks for a light. Bruno carefully offers him a matchbook, rather than the crucial lighter. Guy Haines travels back to Metcalf by a different train, but the same man is sitting to his right in the club car - just slightly further away. Either this guy can travel on two trains at once, or there was a bit too much economy with the bit part actors.