In the meantime you can use the following css workaround that will fix the issue: 

.rcbAutoWidthResizer .rcbScroll {
	overflow: visible !important;
}
 
/*To target IE 6, 7 and 8*/
@media \0screen\,screen\9 {
	.rcbAutoWidthResizer .rcbScroll {
		overflow: auto !important;
	}
}
 
/*To target IE 9 and 10*/
@media screen and (min-width:0\0) {
	.rcbAutoWidthResizer .rcbScroll {
		overflow: auto !important;
	}
}

We are running HTML validators as part of an effort to attain WCAG 2.0 A compliance.

Among the errors we are seeing is

The summary attribute is obsolete. Consider describing the structure of the table in a caption element or in a figure element containing the table; or, simplify the structure of the table so that no description is needed.

Here is a snippet of the failing HTML, apparently from the rendering of the RadSocialShare control:

	<div id="ctl00_TemplateBody_WebPartManager1_gwpciNewSocialShareCommon_ciNewSocialShareCommon_RadSocialShare1_compactPopup_C_comboBox" class="RadComboBox RadComboBox_Default" style="width:160px;">
<table summary="combobox" border="0" style="border-width:0;border-collapse:collapse;">

We are currently running version 2013.2.717.45 of the ASP .NET for AJAX controls, but we just tried 2013.3.1015.45 and it didn't fix things in this regard, either.

There is a similar discussion at http://www.telerik.com/community/forums/aspnet-ajax/combobox/table-summary-attribute.aspx , which recommends a jQuery fix.  This fix works in terms of removing the summary attribute from that table, but it does not result in clean HTML when the w3c validator is run against it, so we believe that is not an acceptable workaround.

In the discussion at http://www.telerik.com/community/forums/aspnet-ajax/general-discussions/accessibility-issues-various.aspx , some mention is made of fixing the combo box for the 2012 Q3 release, but it doesn't look like that happened.

When EnableAutomaticLoadOnDemand="true" is set and no Height is specified, the first time RadComboBox is expanded only the topmost 3 or 4 items are visible.
The rest of the items become visible when they are hovered or if the ComboBox is expanded again.

described in this forum thread-http://www.telerik.com/community/forums/aspnet-ajax/combobox/717802-radcombobox-auto-complete-problem.aspx

Customer feedback:

The first combobox below behaves as I would expect to, and resizes when the browser window is resized.  However, the second combobox does not resize when the browser window does.  They use the same skin.  The only real difference that I can see is that I use check boxes in the second one.

When RadCombobox is inside fixed div element and is expanded it doesn't scroll as expected. The dropdown container should stay fixed as the combo's input element.

If we dynamically remove highlighted item from a ComboBox at clientside, afterwards get_highlightedItem still returns that deleted item.
ComboBox takes care of deleting selected item at like 497 of RadComboBox.js. Highlighted item should be taken care of too.

ComboBox keeps the dropdown aligned to either sides of input field.
If due to content overflow e.g in a small frame input field is placed slightly out of viewport then it would be better to check the boundaries if ScreenBoundaryDetection is enabled and keep the dropdown in view and not necessarily aligned with input.

From customer feedback:

Label display style is not taken into consideration when calculating the width of the combobox e.g. if the display style is block, the label and combo should be on separate lines. In terms they should be equally wide.

However currently the combo is squashed to a minimum width.

This feedback item has been raised from this forum thread: http://www.telerik.com/community/forums/aspnet-ajax/combobox/enablecheckallitemscheckbox-all-entry-doesnt-fire-onitemchecked-event.aspx

This issue was first raised back in Sep '11 as it was suggested that 'it was in the plan for a future release'.

I've added this item here in the feedback portal to ensure that it's not got lost

Please check the attached images for more details.