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.
Revealing mistake: The letter which is typed on the Olivetti portable typewriter (certainly an accurate model for this period) is obviously a modern laser-printed letter, not from a manual typewriter.
Chosen answer: Tom & his lover are travelling on a boat. The rich girl, who knows Tom as Dickie is also on the boat. If they were to meet, Tom's false identity would have been revealed, and the lover would have been able to figure out that Tom actually murdered Dickie.
marfbody