Shoyoroku - Case 8: Hyakujo and the Fox [1]

Whenever Master Hyakujo delivered a sermon, an old man was always there
listening with the monks. When they left, he left too. One day, however, he
remained behind. Hyakujo asked him,
  "What man are you, standing there?"
The old man replied,
  "In the past, in the time of Kashyapa Buddha, I lived on this mountain as a
   Zen priest. Once a monk came and asked me, 'Does a perfectly enlightened
   person fall under the law of cause and effect or not?' I said to him,
   'He does not.' Because of this answer, I fell into the state of a fox for
   500 lives. Now, I beg you, Master, please say a turning word."
Hyakujo said,
  "The law of cause and effect cannot be obscured." 
Upon hearing this, the old man became greatly enlightened.

[1]: see case 2 of Mumonkan.