You open up the PSD in Preview, make a minor edit to it such as adding a little text to it or draw a couple of red lines. The intention is to select all, copy, and go to your email app to paste it into a message. You'll just undo the changes or close it without saving later on.
In the background, when you left Preview it autosaved the file and destroyed all the layers. You even quit Preview without thinking, so you didn't get a chance to decline saving it.
Auto Save will be helpful for most users, but it’s also almost guaranteed to make me lose data one of these days. For more than 20 years I’ve been opening documents and making changes that I have no intention of saving. After working in another window or application for a while, I’ll go back, close the document, and click “Don’t Save” when it asks. Lion’s Auto Save would instead write those changes to disk without asking. Sure, the old version would be accessible (for a while), but I’d have to remember to go and revert the document. Or, I suppose, remember to do things the Lion way and duplicate the document before making changes I don’t intend to save. So far I’ve rarely remembered to do that, but I’ve been saved by the fact that the documents I was editing were stored in Git or EagleFiler, which alerted me to the changes I’d unintentionally made. I prefer the old-style Auto Save (like is available in BBEdit), where you can get back unsaved changes if there’s a crash, but it doesn’t overwrite your files unless you say so.