I have two.
1. I was riding on my friend's handlebars, and we were close to stoping but I lifted off my leg while the wheels were rotating and it scraped a small part of my calf. Now I have a small area where its slightly lighter than the rest of my skin.
2. On my arm. In art class, somebody kept on bending a piece of metal wire and when you do that rapidly, it gets to the point when it gets really hot and they touched my arm with it and left a crescent shaped mark on my arm :]