Continuity mistake: When Susan falls over the railing the way she falls from the front is different than how it looks from the shot behind her. Look at her legs and where they are in relation to the railing depending on the shot.
Factual error: The type of Polaroid camera they use in the bedroom of the B&B is an SX-70 type, that spits out the print in front. But the main plot point is that they found the toss-away negative from an older type of Polaroid. There is no way you would get that from an SX-70.
Other mistake: When at sea and during the crew member foc'sle rescue, the ship's crew, including Kevin Costner, are wearing ship ball caps reflecting USS Billings (BB 1369) and the BB represents "Battleship." However they are actually on a Guided Missile Destroyer "DDG 21", the USS Cochrane, as indicated by the hull number 21. (00:15:00 - 00:17:00)