You have ''got'' to hate if when you forget to update a file during Drupal upgrades. After the last Drupal upgrade I've forgot to redploy they old .htaccess file for the Gallery2, which resulted in Gallery being effectively inaccessible from the main site (unless the visitors ''knew'' the alternative link).
Luckily, I wanted to take a look at some of the statistics today regarding page hits and top referrers, and noticed a link to some photo being reported as good ol' ''404'', page not found. Just a couple of clicks later to check why did that happened, it hit me - I forgot (AGAIN!) to take care of the Gallery after upgrade. Happened to me several times already, and each time I seem to forget this step. The worst part is that the thumbnails keep on working, so I don't manage to detect it right away.
Just applied the fix now, so I hope it'll turn out well now.