Here's what I did: Open a page, edit, preview, press re-edit, preview, re-edit (continue for a while). When I then pressed update, Confluence told me someone else had edited the page in the meantime after I started editing and that my changes would overwrite some of the content. I copied my new content into safety and cancelled, opened the page for editing againg and then saving worked fine.

Looking at the page history, the page was last edited in Jan 22nd which is days ago so nobody else was actually editing the content. ( http://confluence.atlassian.com/pages/viewpreviousversions.action?spaceKey=DISC&title=Importing+and+Exporting )

So I guess there's something odd with the temporary persistence of previewed documents which is causing the "check versioning upon save" to compare the documents previewed by the user as opposed to the latest saved version, hence breaking the comparison operation.