-Submitting certain forms (i.e. sending a Private Message, changing the skin, or editing a post) appends an extra '?' to the GET query before the existing '?', resulting in a GET query fail. This does not actually affect the page submission, and it does not happen all the time.
-Submitting a post that is exactly the same (byte-for-byte) as the one above it results in an error. Your double-submission script needs to check authors
None.