I Can't Believe It's Butter - S10-E13
Revealing mistake: When Griff drinks the gravy right before calling "Butter", his glass is empty.
I Can't Believe It's Butter - S10-E13
Plot hole: Griff doesn't know Peg's mom is Butter. He comes to Al's wanting to meet her. Al gathers the family to talk about what to do. They say Griff can't know its Peg's mom, yet they talk out loud and Griff is standing right there while they are sitting at the kitchen table. It's not until the end of the episode he finds out.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.