It is way too easy to accidentally publish a post. It happens all too often that I'm only beginning a post and I do something -- perhaps hit the return button twice -- that makes the post publish.
Just now for example, I wanted to do a post on this Economist article about music, and I copied and pasted the whole article into the "compose" window so I could read it and cut it down to a few choice quotes. I then made some mistake that made a post of the entire thing -- with no identifying link or even quote marks to show that it was copied.
Sorry. And Blogger, please fix that.