Then I couldn't access any admin pages. Meaning of course that I could not then clear my caches... INCLUDING the menu cache to restore it.
Truncating the database cache tables also did nothing to help.
Hindsight tells me I could have investigating rebuilding the registry... maybe.
What fixed it was simply running /update.php
Of course, it helps if you're logged in already as user 1 /admin. I couldn't even log out actually, because that access goes through the menu module as well.