We had an Exchange user with message items appearing in her OWA outbox but which did not display in Outlook. Attempts to delete these items failed and the items kept re-appearing in the OWA outbox possibly indicating some sort of corruption of these items?
Solution: Move the mailbox to another mailbox database setting the BadItem parameter a little higher (I chose 30) and then try deleting the messages again in OWA. This seems to have solved this particular problem for us.
