I just added the Ajax control toolkit to my VS2012 project and then run the project. Got the following error.

'MsAjaxBundle' is not a valid script name. The name must end in '.js'

The name it is referring to is available in the Master page, though it doesn’t make clear as to what the error is.

How to solve it? There are a few changes to be made in the Master page.

   <asp:ScriptManager runat="server"> must be changed to <ajaxToolkit:ToolkitScriptManager runat="server">
    The Assembly="System.Web" reference must be removed from all the ScriptReferences
   MsAjaxBundle reference must be removed

Once these 3 changes are made, your application will run trouble free.