Question:
If one accidentally recited Mezonos on a buckwheat cracker whose correct blessing is Shehakol, what should he do?
Answer:
This matter depends on a well-known dispute among the Poskim.
According to the simple understanding of Admur and the Poskim, and so is the explicit ruling of many Poskim, one who recited Mezonos on a food whose blessing is Shehakol has not fulfilled his obligation, and must therefore immediately say Baruch Sheim Kevod Malchuso Leolam Vaed and then recite the correct blessing of Shehakol before eating.
However, some Poskim, including the Chayei Adam and others, maintain that Mezonos is considered a sufficiently inclusive blessing for foods that provide nourishment and calories, and therefore the blessing may be valid after the fact. According to this view, one cannot simply recite Shehakol again due to the concern of an unnecessary blessing.
Therefore, those who wish to satisfy all opinions should leave their location in a manner that constitutes a halachic interruption (hesech hadaas). For example, if they are in their home, they may step outside the front door and then return. Upon returning, they may recite the proper blessing of Shehakol and eat the cracker.
Those who follow the ruling of the Alter Rebbe do not need to do this and may simply recite Shehakol immediately before eating.
Conclusion:
According to the Alter Rebbe, one should repeat the blessing and recite Shehakol. Those who wish to satisfy the opinions that Mezonos may have already fulfilled the obligation should first create a halachic interruption by leaving the area and then return and recite Shehakol.
Sources:
Seder Birchos Hanehenin 1:3 and 1:10, SHU”A 168:12; Beis Yehuda Ayash (O.C. 41), Birkei Yosef (167:10), Nahar Shalom, Rav Akiva Eiger (Berachos 36b), Ritva Brachos 1:8; 2:17; Birkeiy Yosef 207:1 in Shiyurei Bracha, brought in Shaareiy Teshuvah 207:1; Rashbatz Brachos 40b; Aruch Hashulchan (167:19), Beis Oveid; Pesach Hadvir 202:12; Sdei Chemed Mareches Brachos 1:36; Orchos Chaim 202:2; See Kaf Hachaim 202:9; Kaf Hachaim (202:9), Piskei Teshuvos (206:3), and Halacha Berurah (206:7).
Other Poskim: Derishah (168:2), Taz (168:6), Elyah Rabbah (168:16), Chayei Adam (58:3), Nishmas Adam 2 in name of Kesef Mishneh Brachos 4:6, Biur Halacha 167:10 “Bemakom” and 208 “Ad Shenismaeich”; Shaar Hatziyon 167:11 and 208:31; Ben Ish Chaiy Balak 13; Birchas Habayis Shaar 19; Kaf Hachaim 202:9 and 206:6; Kerem Shlomo 1:66; Halacha Berura 206:7; Based on this view, the Igros Moshe (O.C. 4:40) and Piskei Teshuvos (206:3) suggest that one who mistakenly recited Mezonos should create a hefsek by leaving the location and then recite the proper blessing.