I went on a 30 mile bike ride on Saturday (got rained on, it sucked, whine). Did a full hour and a half of spinning on sunday, plus an hour of weight lifting. Did another 45 minutes of spinning first thing this morning, at which time I started spontaneously bleeding from the knees.
Close inspection showed that I was not, in fact, having some kind of bicycling ecstasy manifesting itself in knee stigmata. Instead I had some road rash from a few weeks back that apparently had retained some grit, that that grit was working its way out from under the skin.
At any rate, I think random bleeding is generally a good sign that one should take a day off.