I upgraded my projects with the latest version of a third party control (happened to be Telerik, but I've seen the problem occur other times as well) and got the ominous error on the BUILD:
'Filename' is ambiguous in the namespace.
The path led to the temporary folders directory. I've read a post where someone deleted all the files in the directory. But that was after I already fixed it.
The fix is simple, use REBUILD instead of BUILD.
BTW, I'm using Visual Studio 2010 and Framework 4.0.