Other mistake: When Steve wants his money back, Al asks how much was Steve bragging about earlier ($5) he could take and make a fortune, instead of giving Steve $5 it's a $20 bill.
Revealing mistake: Peg is making Al a steak. Problem is the burner isn't even on.
Plot hole: Notice when Steve joins the card game. He looks at his cards and says I'm in. The cards are face up on the table! And for some reason the other players have one card facing out where others can see it.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.