Revealing mistake: Peg is making Al a steak. Problem is the burner isn't even on.
Revealing mistake: Marcy dries her dress in Peg's dryer. She comes downstairs with it on looking like it shrank. You can plainly see Marcy put on her belt and pulled up her dress to make it look shorter.
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.
Revealing mistake: At the start of the episode, Peg is making popcorn on the stove. It's making a sizzling sound like she's really popping it but the burner isn't even on.
Revealing mistake: Peg is making popcorn on the stove. It pops even though the burner isn't even on.
Eatin' Out - S3-E11
Revealing mistake: At the end of this episode, Bud supposedly has a lemon frozen to his mouth but he plainly is just biting down on it. Then Al uses his lighter to "unstick" it, but Bud just spits it out.
Revealing mistake: When Bud is at the shoe store with Al, right after Bid says something about the woman with chicken wings, you see the flash from a camera.
Dead Men Don't Do Aerobics - S4-E2
Revealing mistake: When we see Jim Jupiter on his show after gaining weight, you can plainly tell he's wearing padding under his clothes to make him look fat.
Desperately Seeking Miss October - S4-E7
Revealing mistake: In the scene where Al and his Dad (both played by Ed O'Neill) are sitting on the couch side by side watch the lines of the cushion in the middle. Every time one of them moves you can see where the two separate shots are overlaid. (00:14:15)
Revealing mistake: Kelly is standing by the clock on the stairs. You can see Al and Peg's shadow on the wall waiting for Kelly to come up the steps, so they can come down. (00:02:59)
Revealing mistake: When Kelly enjoys her triumph at the middle of the stairs you can see a shadow on the wall right from her - obviously Al stands behind the wall waiting for his cue. (00:19:00)
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)
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.