Other mistake: When the bullies are riding around Bart and Milhouse there are problems with their water bottles. In some frames they vanish, in some others the bullies' far legs are drawn in front of the bottle.
Continuity mistake: In this episode it is shown that Snowball I is buried in their backyard, but in other episodes, notable the one where Sideshow Bob becomes mayor, it is established that she is buried in a pet cemetary.
Continuity mistake: The cats' graves change position. When Snowball II is buried, they're away from the the tree and near the fence. When Snowball III is buried, the graves are away from the fence and Snowball is among the roots of the tree.