User blog comment:DamiedeterJR/Gravity Falls Question/@comment-5643286-20130621163149

I'd say a crossover (P&F), cheesy or too predictable plots (zombies), if a character died (not counting monsters), if Gideon became a main character rather than recurring (no, JK, it's okay if he does), if Hirsch left the show, if they copy another show, if Mabel actually gets a boy and keeps him for more than one episode (it's best to keep it as a recurring gag), or if the twins turned into monsters.

All those are reasons I would quit watching the show.