Build a Better Mousetrap - S2-E15
Continuity mistake: When Al hits his thumb with the hammer when trying to hit the mouse in the wall, he hits his left thumb. In the very next scene, Peggy is wrapping up his right thumb.
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.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.