Revealing mistake: Peg is making Al a steak. Problem is the burner isn't even on.
Revealing mistake: Marcy dries her dress in Peg's dryer. She comes downstairs with it on looking like it shrank. You can plainly see Marcy put on her belt and pulled up her dress to make it look shorter.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.