<p>I am using the <strong>AjaxControlToolkit</strong> in VS2005, and it works fine. I do have some issues though, when I go to some pages I have, then click back, I get this JavaScript error:</p>

<blockquote>
  <p>'AjaxControlToolkit' is undefined</p>
</blockquote>

<p>I have searched MSDN forums, and google, and tried many of the solutions, but none have worked. I have tried, <code>EnablePartialRendering="true",</code> and others. Short of rewriting everything and changing the workflow of my application, is there any way to find the root cause of this, or fix it?
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Is that a javascript error?
I suppose it has to do with back-button support in the toolkit.
And undefined errors mostly occurs because somehow the script that contains "AjaxControlToolkit" doesn't gets properly loaded.
Thing that come to mind:
The order scripts get loaded, does the Toolkit gets priority?
When there are errors in any of the loaded scripts all the other scripts that hasn't loaded yet will simply be canceled and not gets loaded.
See the outputted HTML of the problem page, find links to all the AXD files and make sure you can download them and see valid javascripts inside.
And if you don't already, get
Firefox
and
Firebug
and you should be able to trace to the actual script line that emits the error.
Hope this helps.