Poppy's By the Tree (1) - S2-E1
Revealing mistake: When Al, Peg, and the kids get to their room, notice the "hole" in the blanket on the bed. It's there the entire episode so at the end, the killer can shove the machete up through it.
Poppy's By the Tree (2) - S2-E2
Revealing mistake: When Al is blocking the killer's axe with his suitcase and they struggle for it, it is obvious that the axe is made of rubber, as the handle keeps bending.
Revealing mistake: When the mall Santa lands in the Bundys' back yard, everybody looks toward the back, and when Al looks forward, he cracks a smile although he's upset about the matter. Ed O'Neil admitted he couldn't contain his laughter.
Build a Better Mousetrap - S2-E15
Revealing mistake: Al smashes his finger while holding a piece of cheese next to the hole in the wall. But you can see the hammer wasn't near his finger at all. It hits the wall.
Im-Po-Dent - S2-E19
Revealing mistake: Bud and Kelly put Tang in glasses. Al says "what are you hiding?" They quickly "drink" it but the glasses are empty.
Just Married ... With Children - S2-E20
Revealing mistake: When Al spins Peg on the wheel to win the bowling ball, in the long shots, you can plainly see its not really Peg on the wheel. It's a dummy.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.