1. Shamai and Hillel argued about whether harvesting grapes enables them to become Tamei.
2. When Beis Shamai realized they were the majority, they "pulled out a sword" in order to maintain that majority.
3. The Amora'im give various reasons to defend Shamai's decree that harvesting grapes enables them to become Tamei.
4. They also decreed on that day that an outgrowth from a product of Terumah is considered Terumah.
5. The Mishnah discusses whether one is permitted to start Melachos before Shabbos that will conclude (on their own) after Shabbos begins.
A BIT MORE
1. Shamai: Harvesting the grapes makes them fit to become Tamei. Hillel: Harvesting the grapes does not make them fit to become Tamei.
2. The Gemara says that Hillel sat before Shamai on that day as if he were one of his students. That day was considered as difficult as the day when Bnei Yisrael made the Golden Calf.
3. One reason is that a person might squeeze out a cluster of grapes to see if they are ripe and ready, and that juice will remain on the grapes when they are harvested, making them fit to become Tamei.
4. Even if an entirely new plant grows from the ground from the seeds of Terumah that disappear when planted, this new plant is considered Terumah mid'Rabanan.
5. For example, there is a dispute about whether a person may set a trap to catch animals or fish before Shabbos when it might catch animals or fish on Shabbos. Beis Shamai: It is forbidden. Beis Hillel: It is permitted.