When the countdown finished or gets interrupted, the tooltip will remain stating the latest value. So even if the timer is inactive, hovering the icon might say "56 seconds" or something. That's confusing and probably easy to fix.
Fixed in http://git.xfce.org/panel-plugins/xfce4-time-out-plugin/commit/?id=742018134c75fcdaf0a30b32be4050fe9c4d8b06