"It... depends on the method used. There's two types; ones that attempt to sever the spell, and ones that attempt to counter it. Severing the spell can be very difficult, like trying to shoot an arrow in flight, using another arrow. It's best used to cancel spells that require continuous work, at a distance. In that case, yes, it can cause feedback to the spellcaster, quite a bit, in fact. Cancelling the spell is sort of like dropping something hot into water. The two energies meet, cancel each other out, and nothing happens. Both spells are still going off, they just have no effect, so no feedback."