Revealing mistake: In the scene with the malfunctioning juicer, you can see the actor who plays Billy is about to laugh right before all the juice explodes out of the machine. As that is happening, you can see he has broken out into a full laugh. True, he might be showing some mirth over another one of his father's malfunctioning inventions, but he was about to break out into laughter before anything happens, and his laughter is inconsistent with his frustrated demeanor when Pete walks into the kitchen. (00:22:40)
Continuity mistake: Both the scene of Billy, Pete, and the science teacher looking at the gremlin cocoon, and later when Pete is in class watching the science film, occur on the same day, yet Pete is wearing different clothes.