I encountered recently some mysterious warnings and errors like this one:
warning CS0436: The type .... in 'c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\oracolulmeu\cf1bfaaa\35ba772f\App_Web_k81qkzp9.5.cs' conflicts with the imported type ... in 'c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\oracolulmeu\cf1bfaaa\35ba772f\App_Code.qui8lpjl.dll'. Using the one in 'c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\oracolulmeu\cf1bfaaa\35ba772f\App_Web_k81qkzp9.5.cs'
.. I don't remember the alternate one but it was similar, now, if you happen to see this check your code for the following objects having the same name: master pages, aspx pages, controls (ascx), datasets etc and simply rename of those so all class names are different.