I just wasted a day figuring this out. Basically we have an ASP.net application written in VB dotnet where all the pages inherit from a base page. The base page in turn inherits from System.Web.UI.Page. We use the base page along with some user controls to 'theme' our application.

The application is quite mature so I was a little suprised when I checked it out of source control and it blew up in my face when I attempted to edit one of the web forms in the forms designer. A large message box would pop up complaining that ....

To finish the solution was to remove the Base Page file from the project and copy the contents to new file then add it back to the project then all was well again.

Apparently VS2003 is not very smart when it comes to Design Time / Run Time issues good things are already being said about VS2005 in this reagard but time will tell.