The gemara discusses whether wine can be sprinkled on the fire of the mizbeiach which will cause part of the fire to be extinguished. At first the gemara suggests that extinguishing part of the fire isn't a violation, but concludes that it would be a violation and is dependent on the machlokes R. Yehuda and R. Shimon about דבר שאינו מתכוין. R. Yehuda holds that even though he doesn't intend to extinguish the fire it would be forbidden to sprinkle the wine on the fire, and R. Shimon holds that since he doesn't intend to extinguish it is permitted. This gemara is one of the sources that prove that the machlokes regarding דבר שאינו מתכוין isn't limited to shabbos due to special מלאכת מחשבת requirements, but rather applies to all areas of halacha.
Tosafos in Kesubos 6a quotes the Aruch who proves from our gemara that פסיק רישא דלא ניחא ליה is mutar (we only forbid a p'sik reisha when he wants the result). How could R. Shimon allow sprinkling wine on the fire which inevitably results in extinguishing? It must be that since he has no interest in extinguishing the flame, even though it is a pesik reisha, it is permitted. Tosafos pushes off the proof by saying that even though it is לא ניחא ליה or לא איכפת ליה it would still be assur m'drabonon, and is only permitted by the fire of the mizbeiach because the Rabbonon never upheld their decree to prevent the mitzvah of burning the wine on the mizbeiach.
Rashi on the gemara seems to ask the same question, but answers something very strange. Rashi asks how can R. Shimon permit something which is a pesik reisha, and answers that since he can sprinkle in small drops which wouldn't extinguish the flame, it isn't considered a pesik reisha even when he sprinkles large drops which will definitely extinguish the flame. Normally, the something is not considered a pesik reisha when the result of one's actions is not going to inevitably violate an issur. The fact that one can choose to do something in a way that doesn't inevitably violate an issur, but chooses to do it in a way that does inevitably violate the issur doesn't avoid the pesik reisha problem. How can rashi say that since he can sprinkle small drops, the sprinkling of large drops isn't a pesik reisha? R. Shlomo Eiger (gilyon maharsha) quotes from the bartenura that sometimes the fire is large enough to immediately consume the large drops so that even the large drops don't extinguish the fire. This would be a good approach to explain why it isn't a pesik reisha, but rashi doesn't seem to say this!