When you hit 'save' it presents you with a warning that says something about certain items not being able to be saved (like attachments, which we don't allow here anyway...)
So it's easy to quickly glance at that screen and think your draft was saved, when in fact there is one more "OK" button you need to press.
I've done it myself, too...not a great design point of this software but once you know it you will look out for it.