Mikvaos, Chapter 2, Mishnah 5. We are still within the topic that has run through this chapter: the rule that three lugin of mayim she'uvim, water that has passed through a vessel, ruin a mikveh if they arrive while it still lacks its forty so'im of mei geshamim, rainwater. Once a full forty so'im of proper rainwater are standing there, drawn water can no longer spoil it. Every case therefore turns on two things: when the drawn water arrived, and whether it is regarded as joined to the mikveh at all. In this mishnah both of those points are left uncertain.
The Case
Picture a pit dug out for a mikveh, with sloping walls. Along one of those walls there are small pockets, indentations in the rock or earth, one above the other. Each of these hollows already holds water that had previously been in a vessel, that is, mayim she'uvim, and each one holds exactly a log. Three such hollows together contain three lugin of drawn water.
The Mishnah's words: "Mikveh sheyesh bo shalosh gumos shel mayim she'uvim shel log log", a mikveh that has in it three indentations of drawn water, a log in each one. Now rainwater begins to flow in and the mikveh starts to fill. As the water level rises, it reaches the first hollow and joins with its log of drawn water, then the second, and eventually the third. At every stage of that process the question is: how much valid rainwater was already standing in the mikveh at the moment the third log became joined to it?
The Ruling of the Tanna Kamma
"Im yadua she'nafal lesocho arba'im se'ah mayim kesheiros ad shelo higia lagumah hashelishis, kasher." If it is known that forty so'im of valid rainwater had already collected in the mikveh before the water level reached the third indentation, the mikveh is kosher. At that point only two lugin of drawn water had joined the mikveh, and two lugin do not invalidate. By the time the third log was absorbed, the mikveh already held its forty so'im and was beyond being spoiled by drawn water.
"Ve'im lav, pasul." But if he cannot establish this, if he has no way of knowing whether the forty so'im were complete before the water rose past the line of the highest hollow, the mikveh is invalid. For it may well be that all three indentations had already been reached and their waters were joined to the body of the mikveh while it still held less than forty so'im. In that case the mikveh contained three full lugin of drawn water at a time when it was still vulnerable, and it is pasul.
Rebbi Shimon's Dispute
"VeRebbi Shimon machshir, mipnei shehu kemikveh samuch lemikveh." Rebbi Shimon declares the mikveh valid. His reasoning: since these pockets sit off to the side, running along the wall rather than forming part of the basin itself, the three lugin held in them have the status of one mikveh standing adjacent to another mikveh. In such a case we assess only the water gathered in the mikveh proper, and we need not be troubled by the drawn water sitting alongside it in the wall.
Viewing one mikveh as standing beside another is not an idea Rebbi Shimon invented on his own. The chachamim recognize it too, and it will come up again in mishnayos further on. What divides them here is only whether it can be applied to this particular arrangement. In the view of the chachamim it cannot: the whole excavation is one hollow that the rainwater is filling, so any recess in its sides holding drawn water is mitztaref, reckoned along with the water of the mikveh itself. On that view the three lugin have effectively entered, and the mikveh is pasul. Rebbi Shimon reasons differently: because drawn water disqualifies only derabbanan, a marginal case of this sort may be resolved by treating the recesses as a mikveh of their own resting alongside the main one, and the mikveh remains fit for use.
So the Mishnah leaves us with a mikveh whose validity hinges on a single measurement in a wall, and with a dispute over how much weight to give a rabbinic stringency when the drawn water sits at the edge rather than in the center.