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)
MortugRevealing 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)
MortugRevealing 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)
amycamille1975Revealing mistake: When the generator gets shut off they grab a couple of oil lanterns. However once lit you can see they are light bulbs.
Phil ClarkeRevealing mistake: After the girl escapes the moving truck, and stumbles through the woods, the killer catches up to her, and slashes her neck. It's obvious she's wearing a prosthetic, as it does not match her skin tone.
eaglegrad16Revealing 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.
Chosen answer: Just after the credits, there's a bank sign that says "Friday, 13 - 7:01 - 60 Degrees". Then the DJ says "It’s 7:01 on Friday the 13th of June." and goes on to say some more about it and lists some other bad luck superstitions. At two other points in the movie, two characters (Ralph and Tierney) say "it's Friday the 13th". So there are at least four references to the fact that the movie takes place on Friday the 13th.
Myridon