I compose a post, and push "Submit". After a little while, I am informed that message 77 (or whatever number) does not exist.
If I recall correctly, I have had this happen 3 times. In two of those times, the message showed up a while later. On the third time a message of the indicated number showed up, but it was not my message.
As jar indicated, this appears to be an indexing problem. The index will eventually be rebuilt by a future post. But on an active thread, it might result in two people writing the same message.