I had one myself, and got a report of the same happening to vegasmike; also saw quite a few people double posting. (The post does go through, so the error happens after that.) No obvious clues as to the cause that I saw, but very much worth looking into.
[ Edit: Edited on 13-Mar-2012, at 00:16 by Sander ]
Bumped into this a little earlier myself, thought I had somehow messed up my url, because it as incorrect and as a result was throwing the error. Let's see if it happens when I post this..
Well, no it didn't. Could be something affecting threads with more than 10 posts. Going to go test that theory
Found the problem. There was an outdated file on the server wreaking havoc..