As someone whose main was never mage, I recently switched to playing my mage full time this xpac since he was the only horde I had and wanted a new perspective this time around (The Garrosh Vol'Jin story line certainly helped me make this choice as well.)
But I'm embarrassed to admit that I hadn't quite grasped the whole "timing your pet freeze right before frostbomb ticks off for massive damage" until VERY recently...namely while I was doing Tillers dailies, I had a bomb on a little ookin' monkey pack and laid out a pet nova...and voila...I was simply amazed. And seconds later, filled with shame that my so-called main had been underperforming for so long due to not understanding a core mechanic of my class/spec. :-P
What are you embarrassed to admit about your character, class or general game knowledge?