When Mr. Shoop is trying to convince Wondermutt that Bob has come back by cutting a new doll's head off, he says, "Wondermutt, look, I found Bob!" as he holds up the head. All the while his mouth is visible in the background and it does not move during the entire sentence.
When Ms. Bishop first sees that Pam is staying at Mr. Shoop's place, it is obviously daylight. When Mr. Shoop followers her outside, and Robin warns Freddy about the emotional damage that situation could do to Pam, it is nighttime.
When Chainsaw and Dave barge in to scare the potential substitute teacher, they are supposed to be running two chainsaws. Two-stroke chainsaws produce a lot of blueish exhaust smoke. There is no smoke visible in the shot.