In one of our IIS migration project we noticed a very strange issue i.e. Our website was giving java script errors with AJAX calls.
It was surprising, As we need to remove lots of aspnet extension entries from web.config in order to upgrade to .net 4.0. So first thing comes in my mind was that it was due to .net 4.0 upgrade. Telerik controls were used so another thought was that it is due to Telerik Controls.
We look at lots of configuration option related to AJAX and Telerik and even install ajax extension and telerik on production but we get to real solution after wasting couple of hours.
The problem resolved when we set the application pool property to classic from integrated (Was Set By Default).