Odd behavior when calling cancel in sortable's start event

I was trying to prevent a list item from being moved, so I called cancel() inside the start event, like this:

$("ol").sortable({
    start: function(event, ui) {
        if (ui.item.hasClass('nodrag')) {
            $(this).cancel();
        }
    }
});

This keeps the item from being moved, but a large number of list items are inserted below the item that the user tries to move. In the attached HTML file, try moving the second item you'll see what I mean.

I get this behavior in Firefox 3.6 and Chrome (haven't tried any other browsers).