The voice of Voldemort made an entirely horrifying shrieking sound. "Idiot!" it railed, "fool, useless failure - " and more profanity that Percy had stopped listening to, because his brain had focused entirely on the one fact that he had gleaned from it. Quirrell had fired a Killing Curse at a mirror, reasonably expecting the mirror to shatter. Unforgivables didn't, Percy was rather sure, normally bounce off of anything, not even reflective surfaces, that was how the merpopulation in the Baltic had been eradicated with Killing Curses in the 1100s, they didn't bounce off of water (or mirrors) like normal spells... and apparently this Mirror had proven the exception to the rule. The Killing Curse had bounced, and it had hit Quirrell.