Continuity mistake: When young Al reaches forward with his left arm to take the book that Miss Degroot wants to give to him, she quickly pulls back but young AL still has his arm raised. In the next shot, his hand his down by his side.
Revealing mistake: At the start of the episode, Peg is making popcorn on the stove. It's making a sizzling sound like she's really popping it but the burner isn't even on.
Revealing mistake: Peg is making popcorn on the stove. It pops even though the burner isn't even on.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.