Element is not a known element warning in Visual Studio when using User controls

Question

I have a Visual Studio 2008 project that is showing the following warning when using User Controls, and I haven’t been able to find a solution anywhere.

Answer 1

This can also occur if the element you're trying to add is within the tags of another element that it shouldn't be within.

For Example:

Or in my case, placing an in an and forgetting to put it in the :