While I was developing a dashboard application using Telerik controls, all of sudden, my Visual Studio 2008 started giving the red underline and when I hovered the mouse, I got the following error message.
Element 'xxx' is not a known element. This can occur if there is a compilation error in the Web site or missing web.config file.
I googled and I got lot of results, but none were solving my issue. At last, it turned out to be that it happened due to an Out of memory exception. I realized that I had opened too many applications and that resulted Visual Studio/Telerik to result in the red underlines. Once I closed the applications and restarted the machine, the error went away.