Rama: 87:06 - What is the 2nd case quoted by the Rama in s'if 6?

י עוד כתבו דאין לערב מים שהדיחו בהם כלי בשר עם מים שהדיחו בהם כלי חלב וליתן לפני בהמה דאסורים בהנאה  Furthermore, one should not mix together the water that was used for washing milk dishes with water that was used to wash meat dishes and then give it to an animal because the water becomes forbidden for use.