While I was working with Web Services in one of my projects, I got some compiler errors: The type of namespace foobar could not be found. Actually I checked the line and it looked as the namespace (a Web reference) was not found - but in the Solution Explorer everything looked fine. If you click the namespace identifier which seems to be wrong, a smart tag appears which allows you to extend the namespace to it's full identifier. And there you can see, why the compiler finds an error.

Cool feature if you know that it is there.