Mikvaot, Chapter One, Mishnah Three
The first three sections of today’s mishnah are the same as the first two mishnayot, except in today’s case, the water in the pool was defiled because someone poured unclean water into the pool. In the last section of today’s mishnah, Rabbi Shimon disagrees with the laws in section three (concerning the loaf) all first three mishnayot.
1) If unclean water fell into it and a clean person drank of it, he becomes unclean.
2) If unclean water fell into it and then water was drawn from it in a clean vessel, it becomes unclean.
3) If unclean water fell into it and a loaf of terumah fell in:
a) If he washed [his hands] in it, it becomes unclean;
b) But if he did not wash [his hands] in it, it is clean.
4) Rabbi Shimon says: it becomes unclean whether he washed in it or whether he did not wash in it.
Section four: Rabbi Shimon holds that the water was not fully absorbed into the loaf. There will be at least a drop that remains on the loaf. This drop will defile the entire loaf. The same would be true in the situations in the previous two mishnayot.