There are so many.
Leveling my first toon 1-70 Shaman all in Resto spec spamming lightning bolt and killing one add at time healing me as hell and getting out of Mana every pull. After that i was so bored that i started a warrior (arms) leveled to 70 and decided to start tanking without any knowledge of what to do or how to do it. First entrance in a heroic with some guild mates, charge pulling all the way and getting like 4 packs of mobs at the same time, we imploded as a Supenova. after that i started to study trying to learn my class.
Other random errors: Leveling a DK, equipping as a DK. becoming an officer, trying to talk to most people in wow. a lot of mistakes