I was playing a CN Fighter/Wild Mage. My party was exploring a cave, I was in the rear of the party as we were having to go in single file. The PC at the front shouted, "Trolls!!!" I instantly thought, "Trolls are hurt by fire, my strongest fire spell is fireball." So I set the fireball to go off nineteen feet back from the first troll(because of the twenty foot radius burst). I forgot it had to go twenty feet in every direction. The cave was twelve feet tall by six feet wide, so instead of only encompassing the trolls, it singed off my eyebrows. Took out half the trolls and two of my own party.