(at around 59 mins) Just as Karla and Julie enter the room where the tanning bed is, Karla says, "Come on Julie it's better than a day at the beach girl," yet her mouth does not move until the next shot.
(at around 1h 5 mins) After Julia explains about what happened two summers ago and that she didn't want to drag anybody else into it. Karla mouths, "It's too late for that now," but all we actually hear is, "It's too late for that..."
(at around 34 mins) At the bar, Nancy says, "What you guys want is a Dark and Stormy." She then lists the ingredients as "Vernon's rum, a little ginger beer, a splash of Tabasco". This is not a Dark 'N' Stormy. A Dark 'n' Stormy is a highball cocktail made with Gosling's Black Seal rum (the dark) and ginger beer (the stormy) served over ice and garnished with a slice of lime. The name "Dark N Stormy" and the corresponding recipe is a trademark of Gosling's Brothers Ltd.
When Willis pulls out his hook to kill someone, it makes the stereotypical sound of metal against metal. However, the hook would need to be sheathed to make such a sound; furthermore, a curved hook is unable to be sheathed to make such a sound.
(at around 16 mins) In the film, Karla Wilson wins a radio contest after answering "Rio de Janeiro" to the question "What is the capital of Brazil?" The capital of Brazil is Brasilia, not Rio de Janeiro. But this is explained later in the film (at around 1h 9 mins).
As Julie and her group arrive, the hotel grounds are brimming with other guests. However, when they check in and encounter the hotel manager seconds later, the hotel is totally deserted. Apart from the fact that all other hotel guests seem to have vanished in an instance, it also is highly implausible that even during 'off season' there wouldn't be at least a few guests left.
(at around 32 mins) When Ray is in hospital and his ECG flat-lines everyone panics thinking that he's dying when in fact he's unhooked himself and jumped out the window. ECG monitors have a warning message on the screen similar to "Leads Off" to indicate that the leads are not longer on the patient avoiding panic like it caused in the film.