I notice it at the time, of course. I'm always smacking into something or other, and it always hurts when I do it. But it's not worthy of committing to long-term memory unless it's something truly epic, or truly painful, or truly dumb, or in some way distinct from all the other times I've mashed myself and gotten an owie.
Evidently bumps, scrapes, and bruises that you don't know how you got are indicative of Problems of some sort or other.
Pain hurts. Why remember it?