Well, I know it's a real problem because it happened to me too. I just haven't been able to figure out why it happens. There are 3 ways to send a private message; from reading a post, from the user's profile, and from your own private message section under Account. So far I've tested and examined the code for each with no luck. If anyone can show me a repeatable instance of it happening, then I could fix it easily. Otherwise, all I can offer is "I'm working on it."