Here's the situation:
I have a host page that loads a custom web user control. In my web control I want use javascript and a RadAjaxManager to load up a second web user control. I used this example from Telerik as my guide:
http://demos.telerik.com/aspnet-ajax/ajax/examples/manager/clientsideapi/defaultvb.aspx
However, I kept getting the following error when I would call the client-side "ajaxRequest" method on the RadAjaxManager.
TypeError: Cannot read property 'id' of undefined
I finally figured it out!
On my parent web control I already had implemented a RadAjaxManager. So when I loaded up my second web control with its own RadAjaxManager, the javascript was getting confused as to which manager to use.
Enter: RadAjaxManagerProxy -> http://www.telerik.com/help/aspnet-ajax/ajax-ajaxmanagerproxy.html
After implementing this, my controls worked beautifully!
Hope this helps others, if not myself later down the road when I forget.