My absolute worst experience had to be in Cataclysm when I rolled a Rogue and used Ambush spam up until level 70, and realized just how terrible I was at the class. So I deleted the character because of how frustrating it was to learn the class from scratch.
Basically what it felt like:
"Oh damn, theres more than just Ambush and opening to a Rogue? Alright shouldn't be that hard, let me just go search it up..."