Revealing mistake: When the chain is cut from Al's wrist, you can see and hear that it's plastic not metal. (00:02:28)
Revealing mistake: Kelly is making popcorn on the stove. It starts popping even though the heat isn't even on.
The Hood, the Bud and the Kelly (2) - S10-E15
Revealing mistake: Whenever Al or Jefferson falls off the roof you can plainly see it's a dummy wearing their outfits.
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: 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.
Movie Show - S7-E21
Revealing mistake: When Kelly is eating the boxes of candy you can plainly see they are empty, but she's chewing anyway.
Revealing mistake: Kelly lost all her hair as a side effect from pimple potion. You can see the lines on her forehead that show she's wearing a flesh piece covering her hair to make her look bald. The color is lighter than her face. (00:18:42)
Revealing mistake: When Al slams Kelly's date's head on the banister it moves back and forth indicating it's made of rubber.
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.
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.
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.
Kelly Doesn't Live Here Anymore - S7-E8
Revealing mistake: You can plainly tell that Kelly is wearing a beehive wig at the diner when Peg comes and talks to her.
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.
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.
Revealing mistake: The burglar uses a crow bar to open the sliding glass door. You can plainly see him "pretending" he had to use it when it just opens easily. He didn't really pry it open.
Revealing mistake: When they blow up the scoreboard at first in slow motion, the entire scoreboard is blown up and destroyed. The next shot only a small portion at the top and middle is destroyed, not the entire thing.
Breaking up is Easy to Do (2) - S11-E15
Revealing mistake: When Al falls on the bed with the shoe and puts the hole in the water bed, the shoe doesn't hit the bed anywhere near where the water starts to shoot out from.
Every Bundy Has a Birthday - S7-E3
Revealing mistake: At the park when Al hits the rich man, he goes down like he was knocked out. Soon after, Seven punches the kid. In the background the rich man is actually sitting up on his side and he looks at Al and Seven, then as the camera pans towards him, he lies down on his side like he is passed out again.
Revealing mistake: While they are jousting, there is no one at all next to the stage in the wide shots. Every time they show the stage up close, there are 40-50 people. It changes back and forth the whole scene.
Revealing mistake: When Kelly is at the house of the crazy guy who made the couch, when they drink the cappuccino and he throws the cup, it's clear that the cups are just props as the drink stays in when it falls towards the camera.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.