Revealing mistake: When Spitz is stabbed with the pitchfork, you can see the line where the fake chest is. It's a different color.
Revealing mistake: In the beginning when Michael crawls out of the mine shaft that was just bombed he floats downstream. You see that he floats up to the old man's dock. If you look at him you can clearly see that he is wearing a black lifejacket. The filmmakers didn't even try to hide it under his suit, it is clearly visible on the outside of his jumpsuit.
Revealing mistake: When Michael (dressed up as Mikey) brakes the car when Tina asks him to stop at the store, you see that when he brakes, the rear red lights don't come on. (01:13:25)
Revealing mistake: You can see the knife blade bend when Michael stabs Tina. (01:06:50)