Factual error: Al plants a vegetable garden. 6 weeks later his garden is shown with fully developed vegetables including full stalks of corn. Not possible in a month and a half.
Other mistake: Al is running the water down the rabbit hole. Bud says the ground isn't even wet and asks where the water is going. A few seconds later Marcy is walking through the yard squishing on the ground because it's flooded.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.