Mikvaot, Chapter Four, Mishnah One
1) If one put vessels under a water-spout, whether they be large vessels or small vessels or even vessels of dung, vessels of stone or earthen vessels, they make the mikveh invalid.
2) It is all alike whether they were put there [purposely] or were [merely] forgotten, the words of Bet Shammai.
a) But Bet Hillel declare it clean in the case of one who forgets.
3) Rabbi Meir said: they voted and Bet Shammai had a majority over Bet Hillel.
4) Yet they agree in the case of one who forgets [and leaves vessels] in a courtyard that the mikveh remains clean.
a) Rabbi Yose said: the controversy still remains as it was.
Section one: Rain falls onto the roof and then comes down the water-spout and fills up the mikveh. Such rain is valid for use in the mikveh. The water-spout does not make it into drawn water. However, if after leaving the water-spout the rain water first passes through vessels it does invalidate the mikveh. This is true no matter what the vessels are, even if they are vessels that can’t become impure. In other words, even though these vessels are not considered to be vessels such that they are susceptible to impurity, they are considered vessels such that they turn rain water into drawn water.
Section two: According to Bet Shammai, it doesn’t matter how these vessels got there. Even if he forgot them underneath the water spout, the water collected in them will invalidate the mikveh.
Bet Hillel holds that if the water was not drawn with intent, then it doesn’t disqualify the mikveh (see 2:6-7). Therefore, if he forgets the vessel under the spout, the water that flows through it does not disqualify the mikveh.
Section three: Rabbi Meir says that the sages gathered together and ruled in favor of Bet Shammai. Since the halakhah is usually according to Bet Hillel, this seems to have been an especially memorable occasion (see also Shabbat 1:4).
Section four: Bet Shammai agrees that if he leaves vessels in the courtyard and they fill up with water that such water does not count as drawn water, for he certainly did not intend to draw water in such a manner. In contrast, if he forgot the vessels underneath the water spout, Bet Shammai fears that when he left the vessels there he intended to gather water. He only forgot them there at a later point. This is as if he put them there intentionally, therefore they invalidate the mikveh.
Rabbi Yose disagrees with this. He holds that even if they were forgotten in the courtyard, Bet Shammai still holds that the water invalidates the mikveh.