Mikvaos, Chapter 2, Mishnah 8. The background here is the fundamental principle of our tractate: a mikveh must be formed of mei geshamim, rainwater that arrives on its own, and not of mayim she'uvim, water that was gathered into a receptacle or brought along by human hands.
The case before us is a workman: hasaid, the plasterer, who was down inside the pit of the mikveh coating its walls, presumably so that the water should not seep out. Sheshachach atzits babor, he forgot a pail of his there in the pit, and venismalu mayim, the water of the mikveh flowed into it and filled it. He now has a vessel sitting in his mikveh full of water. What is the status of that water?
The Ruling of Rebbi Eliezer
Rebbi Eliezer begins with the case in which the mikveh's water rises past the top of the pail: im haya hamayim tzofim al gavov, kolshehem, so long as the water covers it by even the tiniest amount. Here the ruling is yishaver, he smashes the vessel, and its contents empty out into the surrounding water. That water never acquired the disqualification of she'uvim, and for two reasons. The pail landed there through forgetfulness, sheshachach, with no plan of collecting anything in it, and, more importantly for Rebbi Eliezer, the liquid it held was in contact with the body of the mikveh from the first moment, forming one continuous whole with it. Water that never stood apart from the mikveh's own water is not reckoned as drawn.
Ve'im lav, but if the rim of the pail rises higher than the water line of the mikveh, then lo yishaver, breaking it accomplishes nothing for him. Since the mikveh does not yet hold its forty se'ah, breaking the pail at this point would mean introducing she'uvim into the mikveh. Here the water in the pail was standing on its own, separate from the body of water below, unlike the first case where the two were always one.
Divrei Rebbi Eliezer, this is Rebbi Eliezer's position, and it is entirely consistent with his approach elsewhere: water that ended up inside a vessel shelo ladaas, without any intention to fill it, is nevertheless she'uvim.
The Ruling of Rebbi Yehoshua
Rebbi Yehoshua omer, Rebbi Yehoshua disagrees and rules bein kach u'vein kach: the relative heights make no difference at all. Whether the vessel's rim protrudes above the surface or lies submerged beneath it, the outcome is identical, yishaver: he may shatter the pail, and what pours out of it joins the mikveh without invalidating it.
Rebbi Yehoshua's reasoning is that the pail was nismala shelo ladaas, it filled with no intention behind it at all. The vessel was left there by accident and took on water by itself; the man never meant to collect water in it. Where there is no such intention, the halachah is that the water does not become she'uvim, and it is not disqualified for use in the mikveh.
The lesson of this mishnah, then, is how much depends on a person's intention. For Rebbi Eliezer the decisive factor is whether the water in the vessel remained joined to the water of the mikveh; for Rebbi Yehoshua the decisive factor is that nobody ever intended to draw this water at all.