Gemara

(Beraisa): In Eretz Yisrael, if one vowed from oil, sesame oil is permitted but olive oil is not. In Bavel, it is vice-versa.

In a place where both are used, both are forbidden.

Objection: This is obvious!

Answer: It is not obvious in a place where one of them is used more. One might have thought that we follow the majority usage. The Beraisa teaches that we do not. Rather, Safek Isur l'Chumra (we are stringent).

Berachos 21a (Rav Yehudah): If one is unsure whether or not he said Emes v'Yatziv, he must say it.

He holds that Emes v'Yatziv is mid'Oraisa.

Rishonim

Rif and Rosh (Nedarim 17b and 6:5): If one vowed from oil in a place where both sesame seed oil and olive oil are used, both are forbidden. We do not follow the majority usage. Rather, Safek Isur l'Chumra.

Nimukei Yosef (DH Safek): Perhaps he intended for the oil found there. He is believed to say that he intended only for olive oil.

Rambam (Hilchos Nedarim 9:5): If one vowed from oil in a place where both sesame seed oil and olive oil are used, and the majority call olive oil Stam 'oil' and call sesame seed oil 'sesame seed oil', both are forbidden. He is not lashed for sesame seed oil.

Beis Yosef (YD 217 DH ha'Noder): Surely he is not lashed for the minority. It is forbidden only due to Safek, and one is not lashed for a Safek.

Question (Lechem Mishneh 10:9): We always follow the majority regarding Isurim. If one vowed 'until summer', he is forbidden until the beginning of the wheat harvest. The Rosh (61b DH ha'Kol) and Tur (YD 220) say that if most people in the area harvest barley, he is forbidden until the beginning of the barley harvest. Why don't we follow the majority regarding oil?

Answer (Lechem Mishneh): Here, he intended only for one time, so we assume that he intended for the majority. Regarding oil, perhaps he intended for both, so Safek Isur l'Chumra.

Poskim

Shulchan Aruch (YD 217:17): If one vowed from oil in a place where both sesame seed oil and olive oil are used, both are forbidden, even if the majority use one of them.

Shach (25): We do not follow the majority because Safek Nedarim Lehachmir.

Shulchan Aruch (OC 474:1): We drink the second cup of wine (at the Seder) without a Berachah.

Rosh (Teshuvah 14:5, brought in Beis Yosef DH va'Adoni): It is proper to bless only on the first and third cups. One who minimizes Berachos does not lose, for the Berachos are not Me'akev. One who increases Berachos loses, for it is a Berachah l'Vatalah.

Rema: The custom of Ashkenazim is to bless on it.

Maharil (Seder ha'Hagadah 28): The Rosh (Pesachim 10:24) says that we need not bless on the second cup. Tosfos says that we must bless. The Rosh says that one may not benefit from the world without a Berachah. This is Safek Isur, so we are stringent (and bless).

Question (Chok Yakov 2): We are stringent about Safek Berachos not to bless! The Maharil himself says so (58:2), and the Rosh (14:5) says that it is better not to bless. Here the Isur to benefit without a Berachah does not apply, for he refrains due to Safek Berachah l'Vatalah. L'Chatchilah one should avoid entering a Safek if possible.

Aruch ha'Shulchan (3): There is a Safek whether or not the Hagadah is a Hefsek. Therefore, we rule that one should eat less than a k'Zayis of Karpas. If he ate a k'Zayis, he should not make a Berachah Acharonah. Likewise, we should not bless on the second cup, and not drink it. Why should a mid'Rabanan enactment of four cups force one to drink Safek Isur, i.e. benefit without a Berachah?! Rather, Chachamim authorize him to bless. This applies to a Berachah Rishonah. Regarding a Berachah Acharonah, it is a Safek, and Safek Berachos Lehakel.

Mishbetzos Zahav (1): If one intended that Borei Peri ha'Gafen of Kidush should exempt the second cup, he should not bless on it. The Isur to benefit without a Berachah is mid'Rabanan, and some say that the Isur of Berachah l'Vatalah is mid'Oraisa.

Chelkas Yo'av (OC 58): The Rambam holds that one is Yotzei Berachos through thought. Even though the Halachah does not follow him, the Magen Avraham (185:1) says that if one did not pronounce the words due to sickness or Ones, he was Yotzei. Here he is Ones, for he cannot drink (perhaps the Halachah follows Tosfos), and he cannot bless (perhaps the Halachah follows the Rif)! R. Akiva Eiger (184:4 DH Minei) cites Ginas Veradim, who says that one should think the words of any Safek Berachah mid'Rabanan, to be Yotzei according to the Rambam.

Rav Pe'alim (3 OC 7 DH u'Mah): Many Acharonim say that when one has a Safek, it suffices to think the words of a Berachah. Some say that the Isur of Shem Shamayim l'Vatalah applies even to thought.

Shulchan Aruch (210:2): If one tastes a Tavshil (cooked food), he need not bless unless he tastes a Revi'is, even if he swallows. Some say that he must bless; one who tastes is exempt only if he spits it out. In that case, he is exempt for any amount.

Beis Yosef (DH ul'Inyan): Since the Rif and Rosh agree, and we are lenient about Safek Berachos, we do not bless.

Rema: We are lenient about Safek Berachos.

Magen Avraham (10): I do not consider this a Safek. No Posek explicitly permits swallowing without a Berachah!

Bach (OC 29 DH u'Kevar): Normally, one may not bless mi'Safek, lest it be a Berachah l'Vatalah. Shehecheyanu is different, for one blesses due to Simchah. Even if he was not obligated, it is not l'Vatalah, for he thanks Hash-m It is Reshus (optional) to bless Shehecheyanu on gourds. This shows that one blesses Shehecheyanu due to a Safek.

Rejection (Chasam Sofer OC 1:55 DH v'Hinei ha'Bach): One may not benefit without a Berachah, but there is a Shi'ur to the benefit (for Shehecheyanu). If a benefit it very small, even if one says that it is important to him, Batlah Da'ato Etzel Kol Adam (we ignore his opinion). Some things, e.g. gourds, are Reshus (depend on the person). If he know that he benefits, he is obligated!