Continuity mistake: When Bruno and Guy first meet on the train Bruno orders some whiskies and lights a cigarette. He talks with this in the corner of his mouth until the camera angle changes mid-sentence when it disappears without him moving. (00:05:10)
Factual error: At one point in the tennis match, Guy hits a ball that bounces twice before his opponent can get to it, but it is not called. (01:22:25)
Factual error: When Bruno is on his way to Union Station, he's heading on a bridge away from the Lincoln Memorial, which means he's leaving Washington D.C. for Arlington, Virginia, or in other words, he's going the wrong way.
Plot hole: Farley Granger meets a math professor on a train bound from New York to Washington, and is able to prove as much to the police. The professor got on in New York and got off in Wilmington, DE, while Granger boarded the train in the fictitious town of Metcalf, NJ, and rode on to Washington. Thus, they met between Metcalf and Wilmington. But the police say Granger could have boarded the train in Baltimore, by which time the professor would have gotten off and they never would have met.
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.
Factual error: In the opening scene, two taxis pull up at Washington Union Station with the Capitol visible in the background. The Capitol is not visible from this point.
Continuity mistake: In the scene where Guy enters Bruno's house to kill his father, Guy takes out a flashlight to look at Bruno's map. In one shot, you can only see the map, and the light is very small and circular, but in a wider shot of Guy's upper torso, the light covers most all of the map.
Factual error: Several seconds into the opening of the second scene, the train pulls out of Union Station headed for NYC. A date palm or other palm tree is visible growing on the left side of the train tracks. Palm trees don't grow in Washington DC, but they do in LA where I suspect this footage was filmed.
Plot hole: If Guy was in such a hurry to get out of there, couldn't he have much more easily lost the match in three sets? Surely, a match loss would be a small price to pay to alleviate a murder charge.
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.
Continuity mistake: When Guy and Bruno are having lunch on the train, when the camera is over Guy's shoulder there is no salt visible. One can see both salt and pepper when the camera is over Bruno's shoulder.
Plot hole: Anyone of Anne's wealth and position would have insisted that Guy get advice from a lawyer. Even if Guy somehow refused to go himself, she would have gone to one. There's nothing to lose by going to a lawyer and everything to gain, and there's no way she's never thought of it, given her father's job.