Other mistake: Al eats a slice of pizza. But if you look in the box there's a small piece taken from the center of the pizza. He picked up a full size slice with crust.
Tis Time to Smell the Roses - S7-E23
Other mistake: When Al talks about the man who bought the shoe store he wanted to buy, he says the same guy bought the store he used to work at and got his old job back. However, Gary is a woman who owned the store before this episode and again afterwards for the remaining episodes. So it couldn't have been bought by a man because Gary the woman still owns it.
Other mistake: The Bundys never lock their front door the entire series, but a burglar has to use a crow bar to open the sliding glass door out back.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.