When I follow these steps:
1) edit a page
2) click preview
3) click confirm

Confluence tells me:
"You were editing an outdated verion of this page. Another user has updated this page before you. Your changes against the now udpated version is shown below. Please decide whether your changes should overwrite it or you wish to discard your changes."

It then shows me all the changes I've just made and previewed, even though nobody else is editing the page at all.