Kilayim, Chapter 7, Mishnah 4. This Mishnah and the one that follows deal with a case in which one person's vine becomes intertwined with another person's grain, and the question under discussion is whether this creates kilei hakerem.
The View of the Tanna Kamma:
The Mishnah opens: "Hamesachech es gafno al gabei tevuaso shel chaveiro - harei zeh kidesh vechayav b'achrayuso" - a person who spread out his vine like a covering (similar to the covering of a sukkah) over his fellow's grain has thereby rendered the grain forbidden, and he is also obligated to pay for the damage caused to his fellow by the fact that his grain has become forbidden.
Although the Gemara elsewhere records a dispute as to whether one who causes damage indirectly is obligated to pay, here he is obligated even according to the opinion that exempts indirect damage: since he performed an actual act by covering his fellow's grain with his vine, this is not merely indirect damage. This is the opinion of the Tanna Kamma.
The View of Rabbi Yose and Rabbi Shimon:
Rabbi Yose and Rabbi Shimon disagree and rule: "Ein adam mekadesh davar she'eino shelo" - no prohibition has been created at all here, since a person cannot render his fellow's property forbidden.
The Rav explains: in other areas of Torah law a person can indeed render his fellow's property forbidden - one who pours milk into his fellow's meat thereby creates a prohibition - but with regard to kilayim we learn from the verse that a person does not render his fellow's property forbidden through the growth of kilayim.
In summary: the Tanna'im disagree about the law of one who spreads his vine over his fellow's grain. According to the Tanna Kamma the grain becomes forbidden and the one who spread the vine is responsible for it, and even according to those who exempt indirect damage he is obligated here, because he performed an actual act. According to Rabbi Yose and Rabbi Shimon there is no prohibition at all here, since with regard to kilayim a person does not render forbidden something that is not his.
In the next Mishnah we will continue to deal with the intertwining of one person's vine with another person's grain.