This one caught me today... When loading a crashdump into Visual Studio, the IDE can get its knickers in a twist and cache all the symbol files in the same folder as devenv.exe.
That'd be fine, but the symbol files are cached in folders with the same name as the DLL - so the IDE creates a FOLDER named System.dll (etc) in its own folder - and next time you try and start Visual Studio you get this...
Unpleasant, for sure. Fortunately, I found a Connect discussion that showed the problem, and it's easy enough to fix by deleting all the (obvious) cache folders from C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE.
Just annoying, and I'm sure I've still got more clart to clear from the IDE directory. (sigh)