Visual Studio thinks my controls are missing, but they are not. "The name xx does not exist in the current context"

When working with an ASP.NET application in Visual Studio 2008, my error window often contains a list of errors such as:

The name "txtUsername" does not exist in the current context.

When I double click on the error, it goes to the source, shows the control is highlighted in red, then realizes the error and removes those errors. This is a real pain, because if you have a real mistake, you go and hunt for it.