Sys WebForms Page Request Manager Parser Error Exception

"If you've used the Microsoft ASP.NET AJAX UpdatePanel control, there's a good chance you've hit the Sys.WebForms.PageRequestManagerParserErrorException error."

This article provides some details on what this error is, common reasons it occurs, and ways to avoid getting the error.

 

http://weblogs.asp.net/leftslipper/archive/2007/02/26/sys-webforms-pagerequestmanagerparsererrorexception-what-it-is-and-how-to-avoid-it.aspx