I've been getting this from time to time when posting a quoted reply:
Database Error
Please try again. If you come back to this error screen, report the error to an administrator.
It seems to be hit or miss. I'm using the latest Firefox with Sandboxie if that is useful. Let me preview, just not post. Seems to be only on replies. Only difference I have noticed is it says on preview "Someone has posted to thread, check the <whatever the rest the message is>.
Edited to add:
It actually posted it, twice. Maybe it burped or something.