Bleak Falls Barrows. I was rather fond of using fire magic. Discovering that the pinkish fluid on the floor was flammable was quite the surprise.
"Oh, Mister Undead Skeleton Man, you're no match for my handy-dandy-flame-fingers! Say, this fluid we're standing on isn't reactive, right? Right!"
The explosion launched my sorry behind clear down the hallway and crushed me into the far wall. If the blast and fire hadn't killed me, my spine shattering into seventeen pieces would've.

Got cocky and tried to take on a giant at lev 5... Got smacked back to Whiterun... I LOLed because it's only a game