Peah, Chapter 5, Mishnah 3. The previous mishnah dealt with the ways one must act so that the poor person will not lose what is coming to him because of circumstances that depend on the farmer, the owner of the field. Our mishnah continues with this same subject.
"Ein megalgelin b'tofeach - divrei Rabbi Meir":
A person may not use a rolling device - a sort of water wheel - to moisten and irrigate his field, in a manner that brings too great a quantity of water into the field and ruins the leket lying there. Such irrigation causes a loss to the poor, and therefore Rabbi Meir forbids it.
"V'chachamim matirin, mipnei she'efshar":
The Sages permit it, since it is possible to avoid the loss to the poor: the owner of the field is able to place the bundles of leket on top of something that raises them off the ground, and in that way the poor will receive everything that is coming to them.
The Yerushalmi explains that if irrigating the field ruins the leket, the owner of the field is obligated to pay the poor for their loss. For this reason Rabbi Meir holds that one should not act this way at the outset: if a loss is caused to the owner of the field, who cannot irrigate his field properly, the matter is liable in the end to come out of the pockets of the poor, and the owner of the field will take from the gifts of the poor in order to make up the difference.
A second explanation of the mishnah:
There is an opinion that reads the mishnah in an entirely different way: "Ein megalgelin b'tofeach" - and 'megalgelin' in this context means that one may not harvest the ordinary grain together with the tofeach, a type of legume that is an inferior grade of produce. This is the opinion of Rabbi Meir, because there is a loss to the poor in this: these legumes tend to fall more quickly than the other grains, and it turns out that the poor will receive a disproportionate quantity of tofeach - that inferior legume - instead of the grain that is coming to them. This is the loss of the poor.
The Sages permit it here as well, since in their view the leket may come from the other grain just as easily as it comes from the lesser legume.
In summary: In this mishnah Rabbi Meir and the Sages disagree about an action that is liable to harm the right of the poor - whether according to the first explanation, irrigating with a wheel in a way that ruins the leket, or according to the second explanation, harvesting the grain together with the tofeach. Rabbi Meir forbids it out of concern for a loss to the poor, and the Sages permit it "mipnei she'efshar" - because it is possible to prevent the loss and to ensure that the poor receive the full measure of what is coming to them.