From today's newspaper, in a column explaining the jet stream:
"They are created by temperature contrasts which create pressure difference. Those contrasts create pressure differences and those pressure differences create winds."
Sounds a bit repetitive, huh? Well, it's clear that the writer's original sentence was the first one and then the copyeditor fixed it to be the second sentence (changing passive voice to active voice, and removing the "which"). However, the editor forgot to delete the original sentence. Oops!
As an editor, I edit original sentences like this one all the time. It's an editor's job to improve the text, not introduce errors, however. I try my best to reread what I've just edited, especially in a heavily edited piece.
This Criminal Sentence is a good reminder for me to do just that, and non-editors should reread their work, too!