Other mistake: In the slaughterhouse scene, after Harrigan has injured the Predator with his shotgun and Keyes reappears, the Predator throws its disc, and severs Keyes in half at the waist. We see his legs flop to the ground, and blood pour from above, but his upper half (torso, backpack and weaponry) mysteriously remain hovering out of sight. (According to the director, the MPAA made him cut the footage of the top half of his body hitting the ground because it was too gory, creating this odd error). (01:21:50)
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 the scientists inject the mouse with poison, they put it in a jar. The, after they place it down you can see someone's hand swap the jar for one containing a dead mouse. You can even hear it. (01:17:58)
Revealing mistake: In the opening scene where the zombie kills Johnny. When they fall against the tombstone you can tell it is a dummy.
Revealing mistake: When the lawn mower the heroes send off as a diversion crashes you can see a wire pulling it. (01:01:30)
Factual error: Asystole (the lack of cardiac activity, also known as the 'flatline') is not treated with defibrillation (electric shocks) but with alternating doses of the drugs epinephrine and atropine.
Factual error: When the Security Guard in the pump room gets bitten on the nose, he starts shooting with his revolver (a SIX shooter). Count the bangs. He manages to fire SEVEN times. (00:39:45)
Continuity mistake: When Maria is cleaning up the water on the floor, the rag is red in one shot and then blue in the next.
Deliberate mistake: It would be impossible for anyone to push up on a casket lid six feet underground and make the lid move much less crack enough to get their whole body through, the dirt would fill the box and trap the occupant.
Revealing mistake: In the scene where young Norman kills the horny young girl, as he's talking to Mother, she can be seen moving and even blinking her eyes when the camera's at her back, yet at this point in the movie, she's been murdered and placed back in her bed. Clearly the actress missed the memo.
Continuity mistake: In the strip club, the serial killer puts the dollar bill in the dancer's g-string twice.