Revealing mistake: Obvious dummies when Al tosses Peg off him on one of their separate beds and when Al falls through the bedroom floor to the den.
Chicago Shoe Exchange - S11-E24
Factual error: When Kelly and Jefferson are driving Bud to the chiropractor and puts him on the roof of the car, as they are driving palm trees are lining the road. There are no palm trees in northern Illinois anywhere, let alone lining roads.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.