They are fairly common in martial arts. Most of the time, one has no recollection of how the bruises were collected.
Last Friday, I was paired up with another girl a couple years younger and a couple belts lower than I in sparring class. We went through the kicking drills and the first round of actual sparring. At some point, I blocked one of her strong back kicks with my forearm. I vaguely remember the hit, but thought nothing of it until after class when I was taking the pads off and saw the beginnings of the nasty looking bruise that stands out on my left forearm.
It has blossomed to an ugly purplish-black bruise about the diameter of a golf ball. My co-workers keep asking how I got it. One of the managers even asked me about it in front of most of my other co-workers, which was slightly uncomfortable. I keep having to explain that I take martial arts and ended up blocking a kick the other night. One complained that her arm hurt just looking at mine. It has been an amusing last couple of days, but I hope the bruise heals up somewhat over the next couple days while I am off. Most of my co-workers react with relief when they realize I did not hurt myself at work.
Amazingly, no customers have said anything about it. I figured they would be the worst about it.