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.