Here's one I've gotten a number of times over the last few weeks:
Sorry, SMF was unable to connect to the database. This may be caused by the server being busy. Please try again later.
I wait a few minutes, and poof, back to the site again.
(Oh, BTW: I'm still getting the double-post error 3-4 times per day. If I reload the page, though, it lets me post)