Revealing mistake: When the bailiff pours rum into the judge's jar, it's obvious that there's nothing coming out of the flask. (00:13:05 - 00:14:10)
Revealing mistake: When Al slams Kelly's date's head on the banister it moves back and forth indicating it's made of rubber.
Weenie Tot Lovers & Other Strangers - S5-E18
Revealing mistake: When Peg opens the last weenie tot box, notice that only a ticket falls out. Meaning all the boxes had no tots in them just tickets.
You Better Shop Around (2) - S5-E22
Revealing mistake: When Jefferson starts to shove the 'regulation Foodies cart,' it's very obvious that the actor makes the shaking movements of the cart by himself. (00:08:05)
You Better Shop Around (2) - S5-E22
Revealing mistake: When Al 'bowls' Jefferson out of the way you can see the stuntman jump high and fall back into some kind of holding wire. It's an annoyingly bad stunt. (00:19:45)
You Better Shop Around (2) - S5-E22
Revealing mistake: Marcy is stacking cartons of eggs in her arms. She turns around and a carton falls on the floor making a busted egg sound although the cartons are obviously empty.
You Better Shop Around (2) - S5-E22
Revealing mistake: When Marcy is stacking the cartons of eggs in her arms, you can plainly see that they are empty cartons.
You Better Shop Around (2) - S5-E22
Revealing mistake: Watch Jefferson and Marcy's feet at the end. They start to trip before the turkey even hits their feet.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.