Ok- Fotoguy's broken lens example is more extreme even than I was thinking - Jim Galli once had a rare Cooke (or Pinkham & Smith, or some such portrait lens) that the front element looked like someone hit it with a hammer, but it still made excellent images so long as you zealously shaded the front to avoid flare. I think he had it in the classifieds here, or maybe it was a thread - search the forums for it.