I do the same thing, I make little "what if" changes just to see what might happen so I understand and I have been tripped up like that a few times..  I think the default autosave time is 10 minutes, so you may have just hit it at the right time.

Do your returns always have a state involved?  If you make it a habit to always switch over to the state prior to exiting when youve been tinkering around with a return it will always ask if you want to save prior to exiting.