In the Harry Potter books, George Weasley is hit with the killing curse in the ear. His ear is blown off but he is fine. How is this possible? It hit him but didn't kill him. How does that work?
He was not hit by the killing curse. He was accidentally hit by Sectumsempra, by Snape who was aiming for another Death Eater.
Now Harry was flying alongside Snape on a broomstick through a clear dark night: he was accompanied by other hooded Death Eaters, and ahead were Lupin and a Harry who was really George ... a Death Eater moved ahead of Snape and raised his wand, pointing it directly at Lupin’s back –
‘Sectumsempra!’ shouted Snape.
But the spell, intended for the Death Eater’s wand hand, missed and hit George instead –
Deathly Hallows - page 552 - Bloomsbury - The Prince's Tale
Slytherincess's answer is correct, but the quote contained therein doesn't explicitly state that it was the Sectumsempra that cut off George's ear. There is, however, a different quote which makes it more explicit. Back in the beginning of Deathly Hallows we have the following conversation:
"He's lost an ear," said Lupin.
"Lost an — ?" repeated Hrmione in a high voice.
"Snape's work," said Lupin.
"Snape?" shouted Harry. "You didn't say —"
"He lost his hood during the chase. Sectumsempra was always a specialty of Snape's.