Other mistake: Tom Ripley's ticket says first class in the limo, but when he boards, the rich girl says she saw his stuff in the third class stack. Dickie also says that he's a third class mooch, but according to his ticket he rode first class, so that'd make him a first class mooch, not third. His cabin is never shown. (00:07:20)
Continuity mistake: In the scene where Tom is analysing Dickie's handwriting, Tom is smoking a cigarette and is shown taking a puff. The camera suddenly cuts to behind Tom's head where he has moved his arm & the cigarette - there is not enough time for him to do so. (00:28:24)
Continuity mistake: You can see the red in the water on the left side of the boat. This was before Ripley murdered Dickie. (00:53:14)
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. (01:21:00)
Continuity mistake: When Tom is taking Freddy's body out of his building, Freddy's coat is buttoned to the top. When Tom puts Freddy in the car, his coat is suddenly undone and blood is visible on his shirt. (01:28:00)
Continuity mistake: Tom has a scrape on his face as he is typing the "suicide" note in the apartment. But as he is leaving and peering around the corner of the building, the scrape is gone. (01:38:00 - 01:44:00)
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. (02:00:00)
Factual error: A copy of Miles Davis' album "Tutu" is sitting on a counter. Odd since the recording wasn't made until 1986.
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.
Visible crew/equipment: When Dickie and Ripley are scootering down a little Italian hillside, you can see the shadow of the whole film crew against the hill.
Continuity mistake: When Marge gets out of the water and climbs the stairs on the sailboat her swimsuit is blue. But when she is on board and we get a close up it is red.
Factual error: When Dickie and Ripley go to the first jazzclub together and Dickie and the Italian singer invites Ripley to join them on stage, the microphone they're using is entirely misplaced. That type of microphone wasn't even on the market yet.
Revealing mistake: When Ripley is leaving New York, there is the shot of the ship sailing out of the harbour. Check out the water - one layer has been 'cloned' on top of another.
Revealing mistake: During the scene on the boat while Ripley is reading the book, Paltrow gets out of the water and walks to where she is leaving a trail of wet footprints. If you notice there is already one set of footprints that have not dried from an earlier take.
Continuity mistake: When Freddie is driving the boat, the direction keeps changing from shots to shots. Dickie told him to keep the same direction.
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.
Factual error: The ship on which Tom sails from Venice to Athens is shown from above at sunset. The sun is to the front left of the ship. That means that the ship must have been sailing West, which is hard (and weird) to do on that route.
Continuity mistake: When Tom and Dickie enter the jazz club in Naples, Dickie's hat keeps appearing and disappearing from his head. (Within the film this is a matter of continuity and thus a mistake, regardless if there is an explanation on the music video.)
Factual error: The scene where they are in the cockpit of the sailboat shows StayLock rigging, which was not invented until the 80s.
Visible crew/equipment: When Dickie, Tom and Marge are coming out of the Venice Railway Station via revolving glass doors, the cameraman is visible in its reflection on their left when it closes.
Answer: After running into Meredith, Tom's plan was to stay in the cabin with Peter the entire trip to avoid them seeing one another. But after Tom makes that suggestion, Peter tells Tom that he seen him kissing Meredith. He couldn't avoid or kill her since Peter already saw her. Family members also saw Tom with Meredith. If Meredith suddenly vanished or was killed I'm quite sure there'd be an investigation. Peter was the only option. No one is on board to report Peter missing. Once the boat docks, Tom will be long gone before they discover Peters body. Unlike Meredith, her family would be looking for her immediately since they're traveling together. I'm quite sure he'd much rather have killed Meredith, she meant nothing to him.