Half a dozen times in the last week, I've seen a reference to someone racking their brains. Each time, I've thought "No, no, no, that's spelled wrack; why didn't a proofreader catch this?"
But then I went and looked it up, and found that both spellings are synonymous; and furthermore, that wrack in this sense seems to have appeared later than rack in this sense.
So I was just plain wrong. But I figured I'd let y'all learn from my mistake.