System.Web.Mobile getting Application_Error throw on .NET 4.0

Just had an strange issue.

User updated ASP.NET page that was created on Visual Studio 2008 to Visual Studio 2010. In the process the project got update to .NET 4.0.

One of the pages was inheriting from System.Web.Mobile, however when we hit this page we would get a message talking about the dll for the System.Web.Mobile was not found or one of its dependencies was not found.

I went inside the project and change it to .NET 3.5 and this fixed the issue.

Now if you need to do .NET 4.0, System.Web.Mobile has been deprecated, so you need to use MVC instead.