<script src="jquery-1.2.4a.js"></script>
<script src="jquery.ui-all-1.5b2.js"></script>
<style>
.test { width:200px; height:200px; margin:10px; background-color:#68BFEF; }
</style>
<script>
$(document).ready(function(){
    for (var div,i=1; i<=5; i++) {
        div += '<div id="'+i+'" class="test">'+i+'</div>';
    }
    $(div).appendTo("body");

    $('.test').draggable({
        helper: 'clone'
    });
});
</script>
scroll to botton, drag the last div, you will see the bug.