Continuity mistake: When Kelly blows up the car near the end, when they show the car before it explodes, you see someone walking on the right side behind the car. Half a second later the car explodes and now someone is walking by the building on the left side of the screen. There wasn't anyone there before.
Deliberate mistake: When the family is trying to get money from Al at the start of the episode, he throws it on the ground making the rest of the family to dive on it and you can see Bud get it, then throw the bunch of money in the middle of Kelly, Peg and Bud so they can all get to it instead of taking it all. (00:02:10)
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.