var handle = Y.Event.onAvailable('#notYetOnThePage', function () {
alert('the element is on the page');
}, Y.Event, true, false);

handle.detach();

The alert will still occur when the element is added to the page.

This call to onAvailable is used by event and at least mouseenter/mouseleave and typing-pause to defer the subscription of events to elements when Y.on is passed a selector string that finds no
elements. Though the use in these cases may be flawed (see bug #2528236), calling detach should at least clear the pending function from Event's _avail array.