Me and my friend have been levelling prot warrior/holy pally combo for a while, doing mainly instances. Doing SM, we got a mage who would use CoC, blizzard, and arcane explosion when we had a single or double mob pull, but would frostbolt spam if we had 3+ enemies.
Then we hit RFD, and got a mage who had a mana pool higher than my friend's pally (who, excepting that mage, has had the highest mana pool of anyone in any group we've done) who managed to go OOM every fight even though he's frost. He pulled off me fairly easily (since he focused on burst damage) and like the guy above, would use a lot of AoEs on single-mob pulls.
Then me and my friend figure we'll switch it up, I'll heal and he'll tank, so we make a pair of druids. Going through RFC, we get a mage that would body-pull, would polymorph the WRONG target (and we needed CC since my friend had no AoE threat generation yet), and would pull marked targets into combat even if we got lucky and they didn't get pulled on the initial pull (i.e. 4 targets close to each other, we mark all 4, but only 2 get pulled, he'd grab the other two).
I know instances at this level are where you're supposed to learn, but still...why would someone use an ability designed against multiple mobs on single targets, and vice versa? Why would someone purposefully make the fight harder (but not faster)?
/endrant