In the times of the beit hamikdash, if a person accidentally transgressed a sin whose intentional transgression would entail karet, he had to bring a korban chatat. If one was unsure if one unintentionally violated such a sin one had to bring a korban asham talui, a contingent sin offering. We don't have sacrifices today. However, we learn from this halacha that even in a case of doubt the accidental sinner is still somewhat culpable.
There is a discussion among various rabbinic authorities how to deal with a case of doubt regarding a mitzvah or aveirah. Generally the rule is "Safek d'oraita l'chumra." One must be stringent and err on the side of caution in regard to a Torah commandment. But, "Safek d'rabanan l'kula." One can be lenient for a rabbinic commandment.
Many authorities are of the opinion that if it wouldn't be a significant hardship to go back and repeat the rabbinic mitzvah, ideally one should repeat it. Regarding blessings, however, te halacha changes. All blessings are of rabbinic origin, with the exception of bentching (Grace after Meals) and the blessings on the Torah. But still this rule does not apply. The rule regarding brachot is "safek brachot l'hakel." In case of doubt one may not make a blessing again because an unnecessary bracha would violate, "Lo tisa shem elokecha lashav."
The Gemara says a blessing may be said in any language. Therefore some achronim suggest that if there's a doubt one should say the bracha in a foreign language. Other authorities disagree. If it can count as a bracha it can also be a violation of "Lo tisa." Most contemporary authorities side with this opinion.
There is a requirement to recite a minimum of 100 brachot a day. The end of the book of Shmuel recounts how 100 Jews were dying every day in a plague. To counteract this, King David enacted that every Jew should recite 100 brachot each day. The Navi relates that after the Jews began doing this the plague stopped.
The Gemara mentions another source for this enactment. In Parshat Eikev the Torah says, "V'ata Yisrael mah Hashem elokecha shoel m'imcha." What does Hashem ask of you? The Gemara says mah can be also read as meah (100). Hashem asks of us to fear Him, to cling to Him, to love Him and the way to reach this is through reciting the meah brachot.
The Kitzur says that there are 100 curses mentioned in Devarim, 98 specific curses and two general curses. The 100 brachot protect us from these curses. On a regular day most of the brachot can be covered by the shemone esrei, which is recited three times a day. Some authorities are lenient and count another 19 with the repetition of the shemone esrei. But most say not to rely on this.
On Shabbat there are only seven brachot in shemone esrei. Therefore, one must be careful to make up the additional brachot in other ways. On a fast day it also gets difficult to reach the 100 mark. The Magen Avraham suggests that one say borei minei besamim a few times throughout the day. Some are stringent not to smell pleasant spices on a day of deprivation. But the common practice is to recite borei meini besamim following the opinion of the Shelah whom the Magen Avraham cites.
The Magen Avraham writes that on Shabbat when a person is short on brachot and makes unnecessary blessings (brachot she'einan tzrichot) it is considered a bracha l'vatala (blessing in vain) and one should not do this. Some authorities disagree and say one may generate a need for extra brachot in order to get closer to 100 brachot. However, many other authorities side with the Magen Avraham.
When we hear a bracha, we must respond, "Baruch hu uvaruch shemo" after Hashem's name and Amen at the conclusion of the blessing. Amen appears in the Gemara, Baruch hu doesn't. Therefore, if one is in the middle of reciting a prayer where one may not interrupt, one should not say Baruch hu. This includes the blessings preceding Shema and Shema itself, when one may not interrupt except for some limited Amens such as Amen yehei shemei rabbah, Hakel hakadosh, and Shomea tefilah. From Baruch She'amar through Yishtabach one should not answer Baruch hu but one may answer Amen to any bracha.
