So here is an annoying thing that I was helping a friend sort out. They were getting an error message that had then stopped in their tracks for a day or so. It was something like:
The base class includes the field 'WebUserControl1', but its type (common_WebUserControl) is not compatible with the type of control (ASP.common_webusercontrol_ascx)
So the application was a asp.net 2.0 app but they were converting it from a 1.1 app. They created the file then copied its contents from the 1.1 app. Well a little google-ing found this article.
He just changed the 'CodeFile' to 'CodeBehind' and all was good. Thank God for other people's mistakes.