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)
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)
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.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.