Beitzah, Chapter Two, Mishnah 8. The Mishnah opens: "Sheloshah devarim Rabbi Elazar ben Azaryah matir vachachamim osrin" - three things, on Shabbos and Yom Tov, that Rabbi Elazar ben Azaryah permitted and the Sages forbade.
The three things:
"Paraso yotzah birtzuah shebein karneha" - a halachah regarding Shabbos: Rabbi Elazar ben Azaryah's cow would go out with a strap between its horns, and he did not consider this an unnecessary burden.
"Umekardin es habeheimah b'Yom Tov" - scraping the animal's back and sides with a metal currycomb that has fine teeth, something like a comb.
"Ushochakin es hapilpelin b'reichayim shelahen" - grinding peppers in the mill designated for that purpose.
The strap between its horns:
The rule is that an animal may not go out into a public domain carrying something that is not needed to guard it or to prevent it from causing damage to people. It must be something necessary; but if it serves only as an ornament - an attractive strap - the Sages forbade it. Rabbi Elazar ben Azaryah permitted it, since its whole purpose is to beautify the animal.
The Gemara notes that it was not actually his cow, but his neighbor's cow; and since he did not protest, the cow came to be called by his name.
Scraping the animal on Yom Tov:
Scraping with a metal currycomb is beneficial for the animal, but there is a concern that it will cause the animal to bleed - a labor forbidden on Yom Tov. Nevertheless, Rabbi Elazar ben Azaryah permitted it, based on the principle of "davar she'eino miskaven" - an unintended consequence: the one scraping does not intend to cause bleeding, and it may happen or it may not happen, and therefore it is permitted.
Grinding the peppers:
Rabbi Elazar ben Azaryah permitted grinding peppers in the mill designated for grinding pepper, with no need for a change on Yom Tov. The Sages disagree and require that it be done in an unusual manner: in a very small mill or in a wooden mill, instead of the ordinary mill made of stone.
The view of Rabbi Yehudah:
"Rabbi Yehudah omer: ein mekardin... aval mekartzfin" - one may not scrape the animal with a metal currycomb or with such a comb, because he makes a wound: he cuts the animal and causes it to bleed, and that is a forbidden labor on Shabbos and Yom Tov. But one may curry, that is, use a comb or brush of wood, whose teeth are broader, which does not make a wound and is not likely to cause the animal to bleed. The basis of his approach: an unintended consequence is forbidden. Whenever there is a strong likelihood that the result will occur, even though the person does not intend it, he may not do it.
The view of the Sages:
The Sages were stricter still. They agreed with Rabbi Yehudah that an unintended consequence is forbidden, and therefore forbade scraping - since it is likely that scraping will lead to bleeding; not necessarily, but likely. And they went further and forbade even the currying that Rabbi Yehudah permitted, as a decree lest one come to scrape. In other words, even with a wooden implement, where it is not at all likely to cause bleeding, the Sages forbade it.
In summary: in this Mishnah we learned of three things that Rabbi Elazar ben Azaryah permits and the Sages forbid: the cow going out with the strap between its horns, scraping the animal on Yom Tov, and grinding peppers in their mill. The central point of dispute is the law of an unintended consequence - which Rabbi Elazar ben Azaryah permits, while Rabbi Yehudah and the Sages forbid; and the Sages went further and decreed against currying as well, lest one come to scrape.