Horrific/gory moments
Some of these I can't imagine how they ended up in a kids' cartoon.
At the beginning of one episode - Partners in Slime? - a ghost pulls his own head off his body and his skull falls out of the neck hole.
Ragnarok and Roll - unlike most/all of the other episodes and movies, thousands/millions of people are said to have been killed by the worldwide intended end of the world catastrophes. Maybe this was allowed since things were reverted, and everyone was brought back to life.