I noticed this today - start posting a new answer or a new question. Now close the tab, you'll get a prompt whether to leave the page or stay on it. Choose to stay on the page, and resume writing, you'll see that preview section isn't updated anymore.
This seems to happen on Google Chrome ( v4.0.266.0) but not Firefox 3.5.6. In Opera I wasn't asked to confirm if I want to close the tab ( log as a separate bug ? )