When the bullies put the snake in Courtney's bag, it's brown. When Finley takes it out, it's changed to black, and is also a rubber snake. And when Courtney picks it up, it's real again, and is now yellow.
After Eddie covers him in mud, Hat climbs up the hill and stands beside Eddie. The mud which Eddie just throw on him is suddenly completely dry.
The snake used in the initial prank is cream and orange. The snake pulled out of Courtney's backpack by Mr. Finley is obviously a black rubber one. It then goes back to being the real snake.
At least three different tarantulas are used when the boys try to scare Courtney with the classroom's giant spider.
When Courtney is speaking to the mud monster, the actor underneath the make-up's eyes and part of his face are visible.