It's happening to quite a few people, most notably when they try to start the SotD threads. Some get around it by putting something random in the subject line, posting the thread, and then going back to edit the title. For SotD threads, some have found that it works to space it out like S O T D, or some other variation.
If this happened to you for something that wasn't an SotD thread, then I don't think I've ever heard of that happening to anyone yet. I hope it isn't developing into a more widespread problem!