Disclaimer: I'm not muscling in on JK's turf - just gambolling on it, like a spring lamb, having fun working out the literary and psychological puzzles which she is having fun setting us.
MESSAGE IN A BOTTLE
"Foolish Potter," the snake-man crooned. "Did you think to oppose me? And dear, dear Severus." He gestured, and the wand skittered beyond Snape's reach. "Later, I shall explain my disappointment at your betrayal, at length."
He gazed down at the dazed boy, the crippled man, laughing as Snape fumbled frantically for the second wand, the special one, silently commanding the wood -
Lightning crackled across the Dark Lord's shields as he flung his arms wide to the skies. "Who will stand against me?"
Smoke poured from the wand, piled up, took shape -
"I will," said the shade of Albus Dumbledore.
Snape, of course, used non-verbal magic to cast Priori Incantatum. The story is based on an idea floated by Red Hen on the Yahoo group Loose Canon.