Sob. I just lost about a dozen incomplete/unsaved documents due to a bug in OS X.
Seems that sometimes, for no particular reason, OS X just deletes all your autosave information. (and … there’s plenty of threads about this on Apple.com and elsewhere. No response from Apple so far. Sigh)
No need to comment on this, really; everything that needs saying about an Autosave that doesn’t Auto-save is pretty self-evident.
Only one extra thought: you might as well disable Autosave on any Mac you have access to, at least until Apple fixes the bugs – it seems you *must* use a 3rd party solution instead, so why deceive yourself with the illusion that it’s safe otherwise? :). I wish I’d known this beforehand :(.