Deliberate mistake: When Saint is being dragged through the parking lot with all the exploding cars, there are a few shots where it is obviously just a dummy instead of John Travolta (the actor who played Saint).
Deliberate mistake: Perhaps to avoid getting an R rating or parental complaints notice how in the operating room massacre none of the doctors appear to bleed nor do Doc Ock's tentacles have blood on them.
Suggested correction: The problem is in both versions of the film, basically all of the deaths either happen off-camera, or are things like people being shocked to death or killed by being tossed around and presumably having their backs/necks broken. Most of the deaths would be bloodless or near-bloodless anyways, and the camera simply doesn't linger on the bodies enough to show any blood. This is also why the tentacles aren't really bloody. Sure, it was probably ratings-related, but it isn't technically a mistake.
Deliberate mistake: When Brody confronts and then flees from the Headless Horseman, the horses never break into full gallop (it's little more than a stiff-legged canter), which is odd for a life-and-death chase with the bloodthirsty Headless Horseman in murderous pursuit. This was probably done on purpose, for safety reasons, since the actor playing Brody was performing his own stunt riding at night. To cinch the illusion of a real chase, audio was added of a 4-beat gallop.
Deliberate mistake: When Tiffany is reading the self-help book for recovering addicts, there's a closeup of a page. While the headline for the chapter makes sense, the body text is completely unrelated and is about corporate work culture. (Likely just some generic free text the prop department copy/pasted into the book prop.)
Deliberate mistake: The music in the 50s wasn't, by far, the Shakira pop and hiphop style that the people dance to in the movie.