I’ve run into this error before when using T4MVC, and I posted on the Alt.NET yahoo group at the time. I don’t recall if I ever explained the cause for this, and since I momentarily forgot about the resolution, I’m posting it here again, as much so that I know it will be lodged in my brain.
When using T4MVC, there is a T4MVC.tt file that is added to your Visual Studio project/solution that generates related .cs files to enable T4MVC functionality. If you change a controller to add a method that returns an ActionResult, and add code in the .ascx file to use that method, BUT you forget to right-click on the .tt file and run custom tool to regenerate the related .cs files, you will get this message.
Obviously, this isn’t an in-depth technical analysis of what T4MVC is doing behind the scenes, so I can’t promise that there aren’t other causes for this error message, but in my practice/experience, this is the main cause.