It was a feature added in when the forums were changed to disallow users from PMing until they had made 10 posts.
If you use the back button/forward buttons if you receive that error, then it will not send, for security reasons. If you get the message, you must click back to another part of the forums, even as simple as clicking back to your inbox. Then you must go through the process of sending the message again.
If you try to send a Pm and get that message, you must wait at least 60 seconds before trying again, as the timer resets every time you try to send a Pm and receive the error.
more info here:
http://hydlaa.com/smf/index.php?topic=27315.0lock for duplicate-ness.