Error handling in an AJAX world can often be tricky — especially when AJAX call-backs are taking place and a mixture of client and server code is running within an application. In its most recent release, the control that comes with ASP.NET AJAX now has much more robust error handling and reporting features for you to use. Specifically:
1) You can now handle the “OnAsyncPostBackError” event on the control to catch and/or modify an error message that occurs during the processing of an AJAX postback callback on the server.
2) You can now set the “AllowCustomErrors” property on the control to enable the standard ASP.NET custom error support to be enabled to redirect a user automatically to an error page when an error occurs (even if it is happening during an AJAX postback).