Revealing mistake: When Evelyn is hanging the clothes after washing them, they are already dry.
Revealing mistake: Baca is using a pistol to shoot the zombies towards the end and still has this same pistol at the end. When he's on the ground about to shoot himself to stop from turning, it shows a close up of his pistol which reveals it's a 6mm BB gun. (01:20:15)
Revealing mistake: When Drake's henchmen are chasing Eddie on the street while he is riding his bike on the sidewalk you can see the stunt man driving for him.
Revealing mistake: The covering of snow was not real snow. When the trick-or-treaters are first seen, there are images of footprints (shoeprints) in the snow, but the trick-or-treaters walking over the "snow" do not leave any footprints. (00:44:38)
Revealing mistake: Davey at the beginning of the movie is looking at the newspaper he's delivering to Mr. Wayne Mackey. Just from a glance at the page you can tell that something is off, since the article supposedly about a missing boy appears to be written entirely in first person. Upon closer inspection, turns out that it's filler text from chapter IV of Robert Louis Stevenson' s "Treasure Island". (00:01:30)
Revealing mistake: When Brother Swan goes to blow on the "horn" (which is actually an ocarina), he toots out a few different notes. However his fingers stay completely stationary on the instrument and not moving around on the holes to change the notes. So it would have been just one sound but it plays like 3. (00:29:25)
Revealing mistake: Normally I wouldn't consider a bad CGI effect to be a mistake, but when Elise finds the hidden keyhole in the wall in the basement, the effect is comical if you pay attention. The way it appears as she scratches is pretty unnatural, and the pieces of paint that fall practically look like cartoon pieces if you pay attention. The shadow of her left hand also looks a little too sharp compared to the rest of her (somewhat softer) shadow, revealing it was likely rotoscopped in some way.
Revealing mistake: While they're finally on the rapids, the girl blindfold keeps changing position from her neck to her forehead. (01:40:59)
Revealing mistake: In the beginning when they're having shots at Hellfest, the guy with the gray shirt holding syringe shots has a stain directly in the middle of his shirt between the drawstrings, 5 seconds later it's gone. (00:13:32 - 00:14:03)
Revealing mistake: When Velma sabotages the science project making it explode into purple goo, you can see that the vial of chemicals she tampered with simply spray around clean liquid, and the purple goo falls from above. (00:23:55)
Revealing mistake: When the doctor with the satellite phone runs to the car, he is being chased by another member of the team, and of course the graboid. Look ahead of the graboid. The path it will take is visible.
Revealing mistake: Annie hears the sound of scribbling in the distance and follows the sound into Charlie's room. When she enters at 1:26:13, you see Charlie's doodle book on the bed being written in and the pages flipping on their own. If you look to the left of the bed, there are a few visible wires running from the book towards the wall to turn the pages. In the photos, they can be seen towards the bottom of the nightstand. In motion, it's pretty easy to see the wires bouncing around (from 1:26:13 to 1:26:21). (01:26:13)
Revealing mistake: At the beginning of the movie, the protagonists dump the corpse of an agent in the middle of nowhere. The guy is stone cold dead...but you can see the carotid pulse on the right side of his neck, under the fake blood. (00:03:10)