I ran across this error after pushing out an upgraded .NET 4.0 MVC2 application to its existing IIS website. After some digging around I found this document on MSDN: How to: Upgrade an ASP.NET Web Application to ASP.NET 4
Since the application was previously built on version 3.5, IIS was configured to run as Asp.Net 2.0. Well it turns out IIS runs under the new framework version 4.0. (not like 3.0, or 3.5 when the Application Pool runs as 2.0)
Kind of one those "I should have figured that out much quicker moments"… A simple change to the app pool to target 4.0, and were on our way.