0x800a139e - Microsoft JScript runtime error: ASP.NET Ajax client-side framework failed to load.

You may encounter this error in MVC applications.
Ajax libraries are rendered to the client side as axd file requests, in turn it actually returns javascript files.
MVC url routing blocks axd files by default.
To avoid this you can add the following line in Application_Start method Global.asax.cs/vb
CS

RouteTable.Routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

VB

RouteTable.Routes.IgnoreRoute("{resource}.axd/{*pathInfo}")