STE

++ Caution!
Don't use an autonumbered category in your preview settings. It could throw a non-fatal error that "the page does not exist.

In my testing tonight, I have determined that when STE previews a page, it first creates and saves the page with a random number in the category you defined in your setup (like mypreview:87). Then the page is retrieved and loaded into the preview pane. The problem is that if mypreview is an autonumbered category, the page name will get converted to the next sequential number (like mypreview:5). Since mypreview:87 no longer exists, you get an error message.

I would highly recommend setting up a category dedicated to preview pages and make sure it's not set for autonumbering.