I tried testing it with the old jQuery autocomplete plugin demos and now, I see it again in jQuery UI autocomplete.
The autocomplete displays a long list of suggestions and it has a scrollbar. When I mouseover on a suggestion while pressing the up and down arrow keys, autocomplete returns to the suggestion with the mouseover on it and doesn't go to other results before that item with the mouseover. It doesn't even return to the first result.
It doesn't happen with the autocomplete without the scrollbar.