More dangerous and permanent than a normal injury, spell damage is the worst type of wound a witch or wizard can receive. This damage often leave permanent scars or effects.
The Janus Thickey Ward at St. Mungo’s is devoted to the care of those most seriously injured by spell damange.
Lockhart received spell damage when a memory charm backfired from a broken wand. He was left with permanent memory erasure. Frank and Alice Longbottom were permanently brain damaged by the Cruciatus Curse, cast by Death Eaters. When Bill Weasley was attacked by Fenrir Greyback in the Battle of the Tower, he received permanent scarring which would never heal properly. The spell damage caused by Snape’s accidental attach on George Weasley resulted in a permanently missing ear.
Some spell damage can be reversed, although with difficulty. The purple slashing attack used by Dolohov, the damage caused by the tentacles of the brains in the brain room, and even the Sectumsempra spell caused unusually damaging effects, but each of these could be cured.