Just happened to me. This is not cool. It's at least a bug or serious UI problem, and maybe even a small security issue.
I was working on a draft, and checked 'Keep this post private' then clicked the save button. That oddly made the post Private, and removed it from Draft status (it no longer was in the Drafts list)---in my mind those are two exclusive concepts---one post could be both private and a draft (a private post, but not finished being written).
Then, i unchecked 'Private' on that post and clicked save. I left it like that for a day, not thinking much of it.
When i returned to write more, the post was neither in Private nor Drafts. It was Published! What was basically my rough notes for a post i was working on had been published, and i had never clicked the Publish button!
That cannot be right.
So my rough notes were sitting out in the wild for a day before i realized what happened.