Revealing mistake: In one shot when the boat is pulling away from the waterfall, you can clearly see that the shot is the same one they used earlier in the movie. The water from the waterfall now is going up the cliff! They have just put it in reverse.
Revealing mistake: In the scene when the main character is in the library on the internet, she goes to a couple of websites. When the shot shows the URL up top, it is C:\windows98\mydocuments\etc.html. Instead of actually searching on the web they used a premade html page and accessed it from their computer.
Revealing mistake: When the UFO makes the huge flash that knocks everyone in the cemetery down, one of the police officers kicks a tombstone as he falls and it wobbles back and forth. (00:13:35)
Revealing mistake: When the team descends to the bottom of a hole and one female member is first wounded and then dies. After checking her pulse, the others pronounce her dead and begin moving out. As the last one is leaving, the "dead" actress lifts her head up off the ground and looks left. (01:13:10)
Revealing mistake: When the kids are burying Gus, his supposedly dead face flinches when they shovel dirt on him.
Revealing mistake: In the fight scene at the pier, after Freddy and Jason get thrown into the air by the carriage that smacks into them, they land on the pier in sort of slow motion. Watch Jason's left knee as he lands, you can see a kneepad.
Revealing mistake: When the demon breaks into the jail and the cop is standing behind him. When the camera turns back on to the demon, if you look close you can see the zipper in the left hand side of the demon's suit. (01:08:45)
Revealing mistake: In the visions of Nefertiri scene where Nefertiri is fighting Anck-su-Namun back in ancient Egypt, Evie (Nefertiri) gets knocked to the floor and when she lifts her mask up you can see on her right index finger is a band aid/plaster. Did they have plasters back in ancient times? (01:07:49)
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.
Revealing mistake: When Eric Matthews is smashing his foot, he hits it the first time and it shows a close up. If you watch the small amount of blood when he hits it again they have used the same shot. (00:04:00)