"Two pees in a pot"
I was looking at a former student's MySpace a few days ago and noticed she had gotten married. I clicked through to her husband's MySpace to see what he looked like, and I came to a photo of him and his son that was captioned "Two pees in a pot". I cracked up. The poor guy obviously missed the point of the "two peas in a pod" phrase.
What phrases have you seen misunderstood?