78:8 The laws regarding a defect found in a sefer Torah are discussed in chapter 24. If a defect is found while reading Maftir, the law is as follows. If the Maftir is an obligation of the day, as is the case on yom tov, Shabbos Rosh Chodesh, parshas Shekalim, and similar such cases in which a second sefer Torah is used for Maftir, then the defect is treated the same as in any other Torah reading. However, this is not the case on a regular Shabbos, where the Maftir repeats the end of the seventh aliyah, which is done only for the honor of the Torah. The reason for Maftir on most Shabboses is so that the honor given to the Torah and that given to the haftarah do not appear identical in that the one receiving an aliyah and the one reading the haftarah both bless before and after; therefore it was established that the one reading the haftarah first receives an aliyah to the Torah, to show that it is the main part. Because this is the only reason we read Maftir on a regular Shabbos, we do not take out another Torah if a defect is found then. Rather, we finish reading from this Torah and do not recite a bracha afterward. The haftarah is then recited with its brachos. If the defect is found before reciting the first bracha on the Maftir aliyah, it appears that we should take out another sefer Torah. (Mishnah Brurah 143:23 disagrees and says not to use a second Torah, but to rely upon the one who received the last aliyah for the haftarah.) If there isn't another Torah available, then the one who received the last aliyah should read the haftarah with its brachos. If Kaddish has not yet been recited, it should be said after the haftarah.
78:9 We learned in 24:1 that if a letter is found to be missing or extra, we do not take out another Torah scroll unless the error affects the word’s pronunciation or its meaning. In such a case, even though we don't take out another Torah, we don't use this Torah for more than the seven requisite aliyos, the seventh of whom then reads the haftarah and its brachos. Kaddish is then not recited until after the haftarah and its brachos. This is the rule for a regular Shabbos but when the Maftir is an obligation of the day as described above, then a Maftir aliyah must be read. This Torah is not to be taken out for use at Mincha.