On a live, multilingual (fr/en) client site, I've just update Drupal 7 to its latest version (along with a few modules). Everything went fine, except that, even though the maintenance mode has been disabled, the maintenance page still shows up on ONE specific URL of the public site.
www.domain.com/ -> OK
www.domain.com/fr -> !!! maintenance page
www.domain.com/fr/ -> OK
www.domain.com/en -> OK
www.domain.com/en/ -> OK
(All the other pages are OK, as far as I can tell.)
I've tried clearing all caches multiple times, and I tried running update.php again, with no effect.
Following other answers, I've also checked the database (cache_bootstrap and variables), but as far as I can tell, everything is normal there too.
Problem solved, although I haven't done anything in particular. Maybe it was caused by some third-party caching mechanism (e.g. from the hosting provider). Will look into that if the problem occurs again.