Friday the 13th (1980)

Revealing mistake: When Ned is pretending to drown, Brenda swims out to rescue him. You can see that she and Ned stay underwater for a few seconds, until their cue to emerge. (00:23:30)

Hamster

Revealing mistake: When Steve Christy is leaving the diner, it shows the outside of the diner, where it is raining. It's very obvious that the "rain" is just a sprinkler.

Revealing mistake: When Alice finds Bill killed and hung up on the door you can see Bill blink his eye. (01:08:05)

Mortug

Revealing mistake: When Jack and the kids get up to the cabin you see Steve chopping on a stub in order to remove it. He then gets help from the kids to tip it over but there are no chop marks on it. (00:13:20)

Mortug

Revealing mistake: When Jack is stabbed through the neck, it is blatantly obvious that the neck is fake. You can clearly see the line between the real and fake neck. (00:41:45)

Hamster

Revealing mistake: Toothpicks visible after Alice cuts off Mrs. Voorhees' head. (01:28:10)

Revealing mistake: When the generator gets shut off they grab a couple of oil lanterns. However once lit you can see they are light bulbs.

Revealing mistake: When Alice opens the pantry door to find Crazy Ralph inside, she is looking directly at the camera as if looking at him, but a second later she's looking to her left.