"potions accident, we were doing cosmetic damage healing potions, stuff to clear up spots and the like and someone slipped a unicorn hair into my brew and it blew up. I was smart enough to put a shield around my station but that only stopped thrown ingredients getting through and it contained the potion to only my workstation so only I was covered. whatever new concoction was made that covered me reacted with the remains of some bad Polyjuice I was slipped in second year, I'm lucky it was just my hair and eyes that changed and that I didn't get cat ears" I explain quickly