I was able to reproduce this manually in a WebKit nightly on Windows, but not well enough to come up with a reliable reproduction. It seems like it might be related to where you doubleClick.
One way to reproduce is to change editing/pasteboard/smart-paste-002.html to call doubleClickAtSelectionStart() instead of:
var target = document.getElementById('test');
var x = target.offsetLeft + target.offsetWidth / 2;
var y = target.offsetTop + target.offsetHeight / 2;