Well here was an interesting one. I was working on adding some AJAX capabilities to a few pages. I had added my ScriptManager to my MasterPage and then on each of my pages I needed to, I added the UpdatePanel to that page. Well on one page, while it compiled fine and worked like it should, Visual Studio was showing my an error on the UpdatePanel control as well as each of the controls in the ContentTemplate. The error was;

    Not a known element

And intellisense refused to work. After a bit of research and trial and error, it appears that it may be a bug with ASP.NET AJAX. There does seem to be a simple workaround however. Simply open your masterpage in the IDE and the sub-page will then behave as expected. According to the ASP.NET forums another workaround is to change the asp prefix to something else (atlas perhaps).