Avyctes of Poseidonis
Nobody bats an eye when a novice wizard gets in over his head. But dark mysteries and ancient sorceries can overcome even the wisest wizard. Just consider the case of Avyctes of Poseidonis. He didn't use his power to brutalize a kingdom. He didn't plot to cast the Earth into darkness. He was content to pine away his final years in study.
Avyctes, however, was an addict. His drug of choice? Forbidden knowledge. So while schemes and worldly power failed to interest him, the mysteries of the ancients proved an irresistible temptation.
So Avyctes purchased the oldest, rarest thing he could find: a mirror-bright tablet of the primordial serpent-people. Absolutely no one knew how the thing worked, and the wizard had to call up the ghosts of prehistoric shamans just to figure out which way to read the text.
When he finally succeeded in activating the thing, it summoned a ghastly shadow. At first this pool of blackness just lay on the floor, but the thing crept steadily closer to Avyctes' own shadow, until finally the two dark patches touched. Avyctes' death was horrible, but that was just the beginning. The ghastly shadow made the wizard's corpse its new avatar, twisting the dead flesh into a vessel for its own unknowable desires.
The lesson: Never recite an incantation you don't understand. That's common sense for most accomplished wizards -- at least until scholarly curiosity overpowers caution.