Here is a list of issues with the YUI Country Autofill Combobox Component:

Pressing the Enter key when the country combobox is open will not only select the highlighted country selection, but the form is also submitted. I don't believe this is correct behavior. Take the comboboxes on this page here. Click one one, scroll up and down with the arrow keys and then press the enter key, the selected item is selected and the combobox is closed The enter press isn't passed onto the form. (I lost my data in the User Information form twice. Very frustrating.)

The country choicbox always opens down, no matter how much space there is left in the browser window. Meaning that if there is only space for two countries to be displayed down, the combobox will lengthen the window and cause the scroll bar to get bigger. If the user wants to see the values lower down they will have to scroll the scroll bar. If, they click the scroll bar instead of using the scroll wheel on the mouse the combobox will close.