Revealing mistake: In the library scene when Richie sees Pennywise the clown, all the balloons filled with blood are popping on people who aren't supposed to be able to see them, but one lands on a lady and you can see her jump as the balloon pops.
Factual error: Winter in Antarctica would not have a night and day as portrayed, with the amount of light changing so much during the course of 24 hours.
Continuity mistake: When the car door is ripped off by the cow, it shows the truck with the door intact.
Continuity mistake: When chasing Jamie in the woods, Mikey's headlights go from normal chevy headlights to off-road high beams.
Revealing mistake: When Brom (disguised as the horseman) throws the jack-o-lantern at Ichabod, there is a shot from Ichabod's perspective showing the jack-o-lantern flying toward him. It jiggles from side to side, revealing it was travelling on a wire.
Continuity mistake: In the scene where the Nemesis kills the 13 S.T.A.R.S members, after it blows up the sniper on the roof an additional S.T.A.R.S member can be seen outside the building on the right who wasn't there earlier. He then disappears shortly afterwards.
Factual error: Water can't put out chemical fires.
Plot hole: So he needed the help of a big strong burly guy to flip the safe with the body, end over end to the pit, but he somehow was able to easily get it out of his basement and into the back of his car by himself?
Character mistake: When they're on the island and the INGen helicopters are flying in, Jeff Goldblum takes the binoculars and looks through the wrong end. (00:32:30)
Revealing mistake: When Charley stabs Dandridge in the hand with his pencil, he stabs the hand holding him by the neck. The depth of the wound on Dandridge's hand reveals that the pencil should have stabbed Charlie in his neck and given him a severe wound, if not kill him.
Continuity mistake: In the scene where Jonathan picks up the scarab, it breaks open and the beetle inside starts to crawl under his skin. We see Brendan Fraser tear Jonathan's shirt and cut the beetle out of him. However, in the very next scene, we see Jonathan walking into the temple, his shirt is intact and there is no blood to be seen. (01:37:05 - 01:38:30)
Visible crew/equipment: When Ashley is running down the corridors before escaping back into her medical room, you can see the dolly that The Engineer chasing her is being pushed on. (01:04:50)
Revealing mistake: As Carlos is giving his final goodbyes to Claire you can see in the background automobiles driving down the freeway. Shouldn't these roads be empty rather than having what seems to be a thriving highway? (01:10:00)
Suggested correction: Not a single car in the background.
Revealing mistake: When John Cusack is driving the actress, there is a shot looking over his right shoulder at the dashboard. The speedometer reads 0 mph. (00:08:15)
Deliberate mistake: At the beginning and especially at the end, Julianna Margulies spends extensive time in the ocean, wearing nothing but cargo pants and a T-shirt. Considering the movie takes place just off the Bering Strait, wouldn't she have quickly frozen to death? (01:21:05)
Other mistake: After Michael kills Laurie at the beginning, he hands the knife to the inmate with the clown mask. The next kill is the cameraman for which he uses the tripod ends. A few scenes later they show Michael standing holding a knife with fresh blood on it. He hasn't killed anybody with this knife yet so where did the blood come from?
Visible crew/equipment: At the start of the film, just after the war exercise, when the Sergeant begins yelling at the recruits you can see the outline of his microphone under his T-Shirt. (It's not his dog tags because you can see the shape and dog tags are not shaped like that.)
Deliberate mistake: In the final shot of the film, Jill's head moves slightly when she's supposed to be dead. In the commentary, Wes Craven confirmed this wasn't strictly intentional, but he decided to leave the take in on purpose in case he decided to bring her back in a future installment. But unless a sequel proves she's still alive, it's technically a mistake, albeit a deliberate one.
Continuity mistake: When Mommie is cutting Christina's hair it keeps on changing from long to short several times.
Suggested correction: Nonsense. Not all can be put out with water, some can.
If you did research on fires you will see that putting water on chemical fires makes it spread. Gasoline and oil float to the surface of the water and continue to burn.