Factual error: In the scene in Venice where Marge is leaving on the boat with Dickie's father, there is a shot from Tom's point of view looking out onto the water. In the distance a modern crane can be seen on the left - something that the director Mingella himself points out on the DVD commentary.
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.