Continuity mistake: When Tom, as Dickie, rents a place in Rome, and after Tom kills Freddie, the police are all over him. Marge comes calling and Tom, as Dickie, asks the policeman to ask her to come back later, which leaves Tom alone with the detective. But as we see Marge creeping up the stairs, to see Dickie, suddenly, the policeman is with Tom and the detective, even though the policeman had left and Tom had closed the doors, so the policeman should not have been there.
Continuity mistake: During the opera intermission scene, when Tom first meets up with Marge and Peter, Peter's hair is neatly slicked back. In the next shot, his hair is hanging in his face a little. The next time he's shown, his hair is neat once more, and a moment later it's messy again.
Continuity mistake: A far ways into the film, when Gweneth Paltrow is leaving on the boat, Matt Damon is standing on the dock watching her leave. At one point, it cuts to a far shot showing him pulling his hair back with his hand, but then it shows a shot from behind, and his hands are down at his sides. It's not very likely he could move his hands that fast.
Continuity mistake: When Tom is playing the piano in the apartment, he has both of Dickie's rings on, but when he gets to the door when Freddie is there, he only has the pinky ring on the left hand. The gold middle finger ring is now missing when ne puts his right hand up on the door, but he never took it off.
Continuity mistake: From the viewpoint of a boat behind Dickie's boat "The Bird", when Dickie gives the tiller to Freddie, so he can go below with Marge, there's hardly any wind, both sails are flapping around, but when we get the view from midships, the front sail, which was on the starboard side, is now, suddenly, on the port side and full of wind, no flapping around at all.