Many problematic mistakes in sifrei Torah can now be caught in advance by computer checks, which every shul should try to arrange. The halachot of what renders a Torah pasul are well beyond our scope, but include problems even with one letter. Our answer assumes that a determination has been made that the Torah in question is pasul.
There are four main approaches in dealing with a case in which a sefer Torah is discovered to be pasul during laining. The simplest opinion, held by most Rishonim, is that the entire laining up to that point does not count.[1] Accordingly, we should begin the laining again from the beginning of the parasha with its full complement of aliyot. However, this opinion is rarely followed these days, and the reason begins with the Rambam.
The Rambam[2] validates the beracha recited during an aliya that was read from a sefer Torah that is pasul because the main element of the mitzva is to read the Torah’s content, not to read from a kosher sefer Torah. More strikingly, the Rambam permits reading from a sefer Torah that is pasul with berachot if a kosher one is not available. We do not accept the latter ruling,[3] and there are even indications that the Rambam retracted it.[4] However, all of the accepted opinions rely on his approach to a certain degree regarding situations of b’di’eved.
The Shulchan Aruch,[5] based on the Mahari Bei Rav, rules that whatever was read before the mistake was discovered is valid b’di’eved. However, if the mistake is found in the midst of an aliya, we finish the aliya (including at least three p’sukim[6]) from a kosher sefer Torah before the oleh makes the concluding beracha. Otherwise, we would be relying l’chatchila on the sefer Torah regarding the concluding beracha. Sephardim and a number of Ashkenazic communities follow this ruling.
The Mordechai[7] objects to taking out a new sefer Torah in the middle of an aliya. He deduces from the gemara[8] that when switching sifrei Torah in the middle of an aliya, a new beracha is required. According to the approach that the reading from a pasul sefer Torah is valid, however, a new beracha would be unnecessary, and thus forbidden, at this point.[9] Therefore, he instructs to finish the aliya, if possible, at the point of discovery and recite the concluding beracha on that which was read. If we cannot end the aliya there (e.g., we did not read three p’sukim, or it is too close to a break in the Torah text), we continue reading from the pasul sefer Torah until we can stop. Some important Ashkenazi poskim say that this is the correct and prevalent minhag.[10]
The fourth approach and third major minhag is based on the Rama’s[11] compromise between the Mahari Bei Rav and the Mordechai. The Rama maintains that it is preferable to stop where the mistake is found, as the Mordechai says. However, if we have not yet read three p’sukim and thus cannot end the aliya, we take out a kosher sefer Torah and read from it without repeating the opening beracha.[12] If three p’sukim were read, which normally allows for a concluding beracha, but we cannot stop for some reason, there is a further machloket. The Pri Megadim[13] maintains that the Rama agrees with the Mordechai that it is preferable to read from the pasul sefer Torah until we can stop. However, the Mishna Berura[14] says that in this case as well, the Rama instructs us to continue reading only from a kosher sefer Torah. Unless there is a minhag to the contrary, we suggest following the Rama and Mishna Berura.
It is important to note that if a mistake is discovered in the last aliya or beyond, the Ashkenazic minhag is complicated.[15] Finally, the b’di’eved validation of laining from a pasul sefer Torah also applies to counting the previous aliyot toward the necessary seven on Shabbat, although having seven aliyot from a kosher sefer Torah may be preferable.[16]