If you’re trying to debug by building the MVC source and getting something like this error:
The type 'System.Web.Mvc.ViewMasterPage' is ambiguous: it could come from assembly 'C:\WINDOWS\assembly\GAC_MSIL\System.Web.Mvc\1.0.0.0__31bf3856ad364e35\System.Web.Mvc.dll' or from assembly 'C:\Projects\mvccontrib\trunk\src\Samples\MvcContrib.Samples.UI\bin\System.Web.Mvc.DLL'. Please specify the assembly explicitly in the type name.
Then Steve will help you solve the ambiguous reference problem.