Shabbos, Chapter 1, Mishnah 9. This mishnah deals with a particular case in which the practice was to be stringent like Beis Shammai.
The practice in his father's house:
Rabban Shimon ben Gamliel said: "Nohagin hayu beis abba, shehayu nosnin kli lavan lekoves nochri shelosha yamim kodem laShabbos" - white garments require a great deal of serious cleaning in order to become white again, and therefore in his father's house they were careful to hand them to the non-Jewish launderer at least three days before Shabbos, so as to make sure that the laundering process could be completed before Shabbos. In this they followed Beis Shammai and were stringent according to their opinion.
"Veshavin eilu va'eilu":
The mishnah continues and establishes that on this point Beis Shammai and Beis Hillel are in agreement, for both of them concede that it is permitted "shetto'anin koros beis habad ve'igulei hagas":
"Koros beis habad" - the beams used for pressing olives in the olive press.
"Igulei hagas" - the round weights that were placed on top of the grapes in the winepress.
It is permitted to set them in place before Shabbos and to allow their work to continue on into Shabbos. Even though it would seem that this is a forbidden labor continuing on Shabbos - something that according to Beis Shammai should present a problem - even Beis Shammai concede that in these cases it is permitted.
The reason for the leniency:
The olives and grapes were already trodden and pressed to a certain degree before they were placed in these presses for the process. Therefore any additional liquid that comes out of them because they remain in the olive press or the winepress is considered only a rabbinic labor and not a labor forbidden by Torah law. The Torah-level labor was already done before Shabbos, and from the moment they were initially trodden, whatever comes out of them afterward is only rabbinic. For this reason it was permitted even according to Beis Shammai, since the action continuing on Shabbos is forbidden only rabbinically, and one may allow the pressing to continue on Shabbos.