Revealing mistake: At the start when the mutant is swinging one of the researchers with a pickaxe, as he swings him into the side of a rock face, the rock shakes. (00:01:55)
Revealing mistake: As Bobby and Brenda are loading the bodies into the van, Lynn can be seen breathing.
Revealing mistake: When the dad goes back to the gas station and finds the attendant in the outhouse, when he shoots himself his head blows a split second before the gun is actually fired.
Answer: Pluto puts his hand over Brenda's mouth and Ethel doesn't bother to look over.