Marge Simpson: How did the pig tracks get on the ceiling?
Homer Simpson: Spider-Pig, Spider-Pig does whatever a Spider-Pig does... Can he swing from a web? No he can't, he's a pig. Look out, he is the Spider-Pig.
When Homer drives to the lake to dump the pig crap silo, there are hundreds of 'No Dumping' signs along the way, but on the way back, all the signs are gone, including the ones Homer missed.
When Homer and Bart jump the gorge, the ambulance that crashed into the tree in "Bart the Daredevil" is still there, crashed in the tree.