If you recall the graveyard scene in GoF, one of the things that happened was that phoenix song filled the air in the graveyard. Going by the logic of pure heart/impure heart, the Death Eaters' hearts should be positively festering with impurities, yet they shouted and ran around the impenetrable area created by the Priori Incantatem, trying to get at Harry.
Given the description of phoenix song, shouldn't they have been running for the hills? Especially in Volders' case -- shouldn't he have dropped his wand at once? I'm thinking the magical properties of phoenix song was the reason why JKR used it in that scene, yet why didn't it work both ways?
An oversight? What do you think? Inquiring minds...