If like me you have installed the Visual Studio 2010 RC, you may have discovered an interesting but quite irritating bug relating to the generation of designer files.
Designer files are those very handy little files that allow IntelliSense to work with your ASP.NET applications. For most people this manifested itself when trying to use the AJAX Control ToolKit TabContainer control and the CreateUserWizard control. Also if you tried to upgrade your existing Visual Studio 2008 project to Visual Studio 2010.
Additional information on the bug can be found on the Visual Web Developer Team blog