Thread:Project:Support desk/1062: Duplicate entry '1' for key 'ss row id' (1.18.1)/reply

Possibly only happened once, but that one time it happened it somehow got cached in file cache, and then shown to all subsequent anon users (logged in users don't get the file cache).

In theory file cache isn't supposed to cache errors, but maybe there's something wrong with it (If you're using an older version of MediaWiki, try upgrading. I think some earlier versions of MediaWiki's file cache support was less good then more modern versions of MediaWiki [but don't quote me on that, that could be wrong])