There are many bugs comes in JavaScript while working with Ajax extension in asp.net. There are many helping tools for debugging Ajax JavaScript. The best tools I found are Firebug for Firefox and Web Development Helper for Internet Explorer. These tools integrate with
the browser and let you inspect and debug web pages directly in the browser.
You can download Firebug form http://www.getfirebug.com/
You can download web development helper http://projects.nikhilk.net/Projects/WebDevHelper.aspx