Great question — answering as the UphillTech team:
Check the referring URL and the user agent. Usually these are caused by robots that found a link to the draft or happened to be scanning your site while you were previewing the posts. I wouldn't worry about it unless it was an actual user that was getting the error.
Full answer by Chris_O: https://wordpress.stackexchange.com/a/623 (CC BY-SA 2.5).
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