Calendar drop refused on occupied cell ?

I have a Calendar application with a drag and drop feature and it works quite well globally.
However, I found out that we you drag and drop an object on an already-occupied-cell nothing happens.

public void drop(DragAndDropEvent event)


is not called whereas the drop condition is

public AcceptCriterion getAcceptCriterion()
return AcceptAll.get();

I’m running Calendar 1.2.1 with Vaadin 6.8.1/Liferay on a Firefox 17.0.1
Is it the expected behaviour or did I find something weird ?

Thanks for the help