I Can't Believe It's Butter - S10-E13
Revealing mistake: When Griff drinks the gravy right before calling "Butter", his glass is empty.
Guess Who's Coming to Breakfast, Lunch and Dinner - S10-E1
Revealing mistake: Near the beginning when Al is looking for his magazines under Bud's bed, a large shadow of someone crosses over him showing someone in production crossed in front of the lighting.
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.
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.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.