The Shulhan Aruch rules (Orah Haim 114:5) that if a person forgot to recite "Mashib Ha’ru’ah U’morid Ha’geshem" in the Amida during the winter months, he must repeat the Amida. However, the Shulhan Aruch then adds a critically important qualification, namely, that this applies only if the individual also did not recite "Morid Ha’tal." But if a person forgot to recite "Mashib Ha’ru’ah" and instead recited "Morid Ha’tal," he has fulfilled his obligation and does not repeat the Amida.
It thus turns out that people who follow Sephardic practice will not likely ever be in a position requiring them to repeat the Amida because they forgot to recite "Mashib Ha’ru’ah." The custom of the Sepharadim is to recite "Morid Ha’tal" during the summer months at the point in the Amida where "Mashib Ha’ru’ah" is recited during the winter months. As such, anytime someone mistakenly omits "Mashib Ha’ru’ah," he almost certainly recites "Morid Ha’tal" in its place; it is highly unlike for a person following Sephardic custom to omit both "Mashib Ha’ru’ah" and "Morid Ha’tal." (Among Ashkenazim, there are many who do not recite "Morid Ha’tal" during the summer months, when "Mashib Ha’ru’ah" is not recited, and this Halacha is thus quite relevant for them.)
The Be’ur Halacha (accompanying essays to the Mishna Berura, by Rav Yisrael Meir Kagan of Radin, 1839-1933) cites an interesting ruling from Rabbi Akiva Eiger (1761-1837) concerning a case of one who forgot to recite "Mashib Ha’ru’ah" (and did not recite "Morid Ha’tal") during Arbit on Friday night. Generally, of course, as mentioned, one who recited neither "Mashib Ha’ru’ah" nor "Morid Ha’tal" must repeat the Amida. Rabbi Akiva Eiger, however, maintained that the Arbit service on Friday night differs in this regard, because the Sages instituted the recitation of "Me'en Sheva" on Friday night, which is a brief repetition of the Friday night Amida. This text makes no mention of rainfall or dew, and it thus seems that adding "Mashib Ha’ru’ah" or "Morid Ha’tal" is not indispensable to the Friday night Arbit service. Rabbi Akiva Eiger thus proposed that one who forgot to recite "Mashib Ha’ru’ah" during Arbit on Friday would not have to repeat the Amida. The Be’ur Halacha appears to accept this ruling of Rabbi Akiva Eiger.
Hacham Ovadia Yosef, however, disagrees, and writes that one should not rely on Rabbi Akiva Eiger’s ruling. He notes that Rabbi Akiva concluded this discussion by remarking, "Sarich Iyun" ("It requires further study"), indicating that he was himself unconvinced of the validity of this theory, and it should therefore not be relied upon. However, Hacham Ovadia writes, one who forgets to recite "Mashib Ha’ru’ah" during Arbit on Friday night does have the option of carefully listening to the Hazzan’s recitation of "Magen Abot." Since this text was intended as a brief Amida prayer for Friday night, one can fulfill his obligation by listening to "Me'en Sheva" in a case where he forgot to recite "Mashib Ha’ru’ah." However, if the individual realized his mistake only after the Hazzan recited "Magen Abot," then, according to Hacham Ovadia, he must repeat the Amida (whereas Rabbi Akiva Eiger would claim that he does not have to repeat the Amida in this case).
In any event, as mentioned earlier, it is unlikely for one who follows the Sephardic custom of reciting "Morid Ha’tal" in the summer months to be required to repeat the Amida after omitting "Mashib Ha’ru’ah."
Summary: One who forgot to recite "Mashib Ha’ru’ah" in the Amida prayer during the winter months must, in principle, repeat the Amida, but if he recites "Morid Ha’tal" in place of "Mashib Ha’ru’ah" then he does not repeat the Amida. Thus, for Sepharadim, who recite "Morid Ha’tal" in the summer in place of "Mashib Ha’ru’ah," it is unlikely that they would ever have to repeat the Amida in such a case, since they presumably recited "Morid Ha’tal." But if a person, for whatever reason, recited neither "Mashib Ha’ru’ah" nor "Morid Ha’tal," he must repeat the Amida. Iif this occurs on Friday night, one has the option of carefully listening to the Hazzan’s recitation of "Me'en Sheva" instead of repeating the Amida.
