Other mistake: When Steve wants his money back, Al asks how much was Steve bragging about earlier ($5) he could take and make a fortune, instead of giving Steve $5 it's a $20 bill.
Revealing mistake: Peg is making Al a steak. Problem is the burner isn't even on.
Have You Driven a Ford Lately - S1-E5
Continuity mistake: Marcy gets two cans of soda for her and Peg to go with their salad. Camera changes shots and it's now two glasses of soda.
Sixteen Years and What Do You Get - S1-E6
Other mistake: Al has his elbow in the caviar yet when he lifts it up out there's nothing on his shirt. Its clean and dry.
Plot hole: Notice when Steve joins the card game. He looks at his cards and says I'm in. The cards are face up on the table! And for some reason the other players have one card facing out where others can see it.
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.
Sixteen Years and What Do You Get - S1-E6
Plot hole: Peg maxes out the credit card buying Al anniversary gifts and hiring a caterer. When Al tries to buy a watch for Peg it's declined. Later on Kelly somehow uses the credit card and is able to buy the expensive picture frame at the same store Al tried to buy the watch.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.