There's an article on cnn.com today about "the only 12 people to have ever stepped foot on the moon."
The correct expression is to SET foot, not to STEP foot. Fixed expressions don't always make sense, but this one actually does: step is an intransitive verb, while set is a transitive verb. Therefore, you cannot "step" your foot; you must SET your foot on whatever it is that you are stepping on, whether the moon or some other, more mundane object.
Thanks a lot, CNN, for giving a faint air of legitimacy to this common mistake.
Yes, I realize there's a certain irony in putting a lolcat in a post where I'm complaining about someone else's grammar. The difference here is that I'm doing it on purpose, and they're not.