I have a big scar on the inside of my left arm, a big curve. I got it back in high school. I was on my way back from a friend's house one night when a guy tried to jump me and take my money.
Now, I was in a martial arts class at this point, and had been for quite some time, so I thought I was some Hot Shit. I wasn't, of course, but that's what I thought. I tried to fight him off, and while I got a couple fairly solid kicks in, he managed to cut my arm before we disengaged.

Not much to the story, I guess, but it's still a pretty big scar.