Like I said earlier Magic is all about intent and visualization, we're taught this from first-year charms to 7th-year transfiguration. It's drilled into our heads constantly but nobody seems to really understand what that means. Accidental magic isn't accidental at all. Magic does not just happen by accident, every single time that accidental magic has occurred the person has wanted something to happen really bad so magic takes the hint and makes it happen. See Rebecca, she hurt me so I wanted to hurt her, the accidental portion of that is the dumb bint blinded herself.