Bloody "nose".
As a young teenager, I was forced to visit my father out of state for several weeks during the summers. One fine day, during one of these summers where I was probably 13 or 14 years old, I snuck off with a young lady I'd been seeing who was several years my senior (but still a teenager herself). We ended up in the basement of her grandparents' house, where we proceeded to do the sorts of things young couples do together in dark rooms. Before the, erm, task was finished, we were interrupted by the sound of an adult coming downstairs. We quickly dressed and turned on the light, acting like the innocent angels everyone thought we were. We were summoned upstairs to be social, and that was the end of our fun for the day.

After I got home, I stopped in the bathroom to take a leak. I unzipped my shorts, pulled myself out, and *almost* passed out from the sight of blood. (You have to keep in mind that at that time I had very little knowledge of such things) I was covered in blood, and my formerly pristine tighty whities now appeared as though they'd been involved in a murder scene. I cleaned up as best as I could, and in my fright, neglected to put said underwear in the garbage, instead putting it in the hamper with the rest of the dirty clothes. Queue a very strange conversation with my stepmother the next day when she discovered them while doing laundry. Thinking quickly, I told her I'd had a bloody nose and grabbed the first thing I could to absorb the blood as I didn't want to drip on the new carpet. She bought it, but I ended up doing everyone's wash (dad, mom, 3 step-brothers) for a week for being so careless as to throw bloody underwear in with the rest of the clothes.
(WillF http://www.flickr.com/photos/silverfischdotnet/, Mon 3 Sep 2007, 3:19, Reply)