After migrating a SharePoint 2010 Content DB to a freshly installed environment my style sheet located in /Style Library suddenly disappeared.
Well, without really thinking it through I created new .css file and renamed it to what it was and should be. I got error saying such file already exists. Say what?
What I didn't immediately remember was that when Content DB backup was taken, it was from a Web Application that was using Classic Mode Authentication.
The new environment instead is using Claims Based Authentication. So, what was going on was that my current user account didn't see the style sheet because the file was checked out to classic-based-me instead of claims-based-me at the time when backup was taken.
In addition that file had never been checked in by classic-based-me so there was no way I could see it when logged in with claims-based-me (from SharePoint permissions perspective they are two different user identities).