I had the event scheduled to record last night. When I looked at it today, it still had a recording icon on it! (Luckily, it was really only 1 minute long.) I tried to stop the "recording" in order to delete it, but it did nothing -- when I went back to the DVR menu, it still had a recording icon. I played it (at which point I saw it was the slate instead of the commercial I was expecting), thinking it would let me stop it then, but when I went back to the DVR menu it now had a playing icon instead of a recording icon, and still no delete option. When I selected the stop option, it went back to having the recording icon! So I rebooted the receiver, and now it can be deleted.
I haven't looked to see if it's scheduled to record again tonight, but as of last night I only saw it once.