Bekhorot 25A

מותר.

it is permitted to pluck the hair in order to facilitate proper slaughter.

ומי אמר רב הכי? והא אמר רב חייא בר אשי משמיה דרב: מסוכריא דנזייתא אסור להדוקיה ביומא טבא!

The Talmud raises a difficulty: And does Rav actually say this? But doesn’t Rav Ḥiyya bar Ashi say in the name of Rav: In the case of this cloth stopper of a jug [mesokhrayya denazyata], it is prohibited to insert it tightly into the spout of the jug on a Festival, because in the process liquid will be squeezed from the cloth, and squeezing liquids is prohibited on Shabbat and Festivals? Apparently, Rav prohibits even unintentional actions, and it follows that it is prohibited to clear hair with a cleaver on a Festival, as one will unintentionally pluck out some hair.

בההיא, אפילו רבי שמעון מודה, דאביי ורבא דאמרי תרוייהו: מודה רבי שמעון בפסיק רישיה ולא ימות.

The Talmud explains: In that case of the stopper, even the lenient authority, R' Shimon, concedes that it is prohibited, as Rava and Abaye both say: R' Shimon concedes that an unintentional act is prohibited if it falls under the category known as: Cut off its head and will it not die, i.e., when a prohibited labor is the inevitable consequence of an unintentional act. In the case of the cloth stopper, some water will inevitably be squeezed out.

והא אמר רב חייא בר אשי אמר רב: הלכה כרבי יהודה, ורב חנן בר אמי אמר שמואל: הלכה כרבי שמעון, ורב חייא בר אבין מתני בלא גברי: רב אמר הלכה כרבי יהודה, ושמואל אמר הלכה כרבי שמעון!

The Talmud objects to the claim that Rav agrees with the opinion of R' Shimon that a regular unintentional act is permitted: But doesn’t Rav Ḥiyya bar Ashi say that Rav says: The halakha is in accordance with the opinion of R' Yehuda, and Rav Ḥanan bar Ami says that Shmuel says: The halakha is in accordance with the opinion of R' Shimon. And Rav Ḥiyya bar Avin teaches Rav and Shmuel’s rulings without mentioning either man, neither R' Ḥiyya bar Ashi nor Rav Ḥanan bar Ami, and says: Rav says the halakha is in accordance with the opinion of R' Yehuda, and Shmuel says the halakha is in accordance with the opinion of R' Shimon.