1. RadAutoCompleteBox with an ObservableCollection set to its ItemsSource

2. The ObservableCollection is populated on the server side once the SearchText of the control changes.

3. The drop down part of the control closes itself even If there are filtered items.

If the AutoCompleteBox is in SelectionMode="Single" editing the text of the SelectedItem will cause the textbox's CaretBrush to go to the end of the text.

If the SelectedItem is initially bound to an object and Tab focus passed through the control the text in it disappears.

AutoCompleteBox cannot be focused on the page load.

If you have RadAutoCompleteBox above RadPanelBarItems and the expanded drop down menu (of the RadAutoCompleteBox) covers them, if you click over some of the drop down items the RadPanelBarItem below gets clicked too.
The fix is available in  Q2 2013 Release.

When an Item is selected, the SearchText property should be updated so it shows the item's text. At the moment it stays with the last entered text. The scenario is valid only for Single mode