In Order of the Phoenix, at the start of Dumbledore's duel with Voldemort he casts a spell that seems to have no effect. What was that spell?
Dumbledore flicked his own wand: the force of the spell that emanated from it was such that Harry, though shielded by his golden guard, felt his hair stand on end as it passed and this time Voldemort was forced to conjure a shining silver shield out of thin air to deflect it. The spell, whatever it was, caused no visible damage to the shield, though a deep, gong-like note reverberated from it – an oddly chilling sound.
‘You do not seek to kill me, Dumbledore?’ called Voldemort, his scarlet eyes narrowed over the top of the shield. ‘Above such brutality, are you?’