A bot posted to it and then recorded the URL to it in some database. This is a shared database of URLs. Occasionally some other bot will look it up in the database, notice it still exists, and post into it. Ok, so it's a pretty crappy trap.
Has anyone encountered the simple captcha recently? Seems a lot more spam lately and I haven't had it show up at all, so it could be broken. I just deleted a spammer that had somehow responded to a message I had already deleted as spam. How the what?