I had some problems with the Accordion control recently. I set it up similar to the sample in the toolkit that is included. It worked great; however, because I have server controls inside the Accordion panes, even though they registered in intellisense, I couldn't use them directly as is showing as available. I researched and found out that you can access the controls via AccordionPaneID.FindControl(), and eventually through AccordionID.FindControl(). However, I was able to get it to work, after calling FindControl on the accordion pane once, by using the control's direct reference, which was strange.
Anyways, this is something to think about and take into consideration when you design using this extender.
When working with the RC version of the ASP.NET AJAX Library, I was getting this error with the updatepanel controls and the scriptmanager control. I followed the Microsoft documentation and renamed everything to the asp prefix, that belonged to the ajax library. However, I kept getting these errors after I upgraded from beta 2 to RC. So what was the problem?
After digging around for a while, I found out on an ASP.NET forums post that the asp prefix is the problem. After changing it to ajax, it works fine. Note that you can have multiple controls use the same prefix (as long as there isn't a name resolution problem), but you can't use the 'asp' prefix. I understand that the microsoft team is getting users ready for when the AJAX library is part of the .NET framework (in the future); however, that was a horrible problem, as it affected how my code got rendered and messed with the designer.