Hello Gretchen Vanderburg,
Thank you for the screenshots.
The __pendingCallbacks array is actually in the ASP.NET AJAX framework code. On the other hand the 'error in the callback' originates in RadComboBox's code. The two may or not may not be related and we need to investigate this further.
We have encountered the second problem a long time ago and did fix it. As I see the info of the forum post you have specified that you are using Telerik.Web.UI Q1 2009 last update, which is an old release, which most probably has the bug. Can you confirm that you are indeed using that version?
Greetings,
Simon
the Telerik team
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