Our tradition has it (Rambam, Hilchos Tefillah, beginning of chapter 12) that Moshe Rabbeinu instituted the reading of the Torah on Mondays and Thursdays in addition to the reading on Shabbos and yom tov. The Rambam wrote in a teshuva that a kosher sefer Torah is not needed for the purpose of fulfilling this takanah deRabannan. However, in Mishneh Torah (beginning of chapter ten, Hilchos Sefer Torah) he writes that a kosher sefer Torah is needed for this purpose. Rav Yosef Karo, in his commentary on the Rambam (Kesef Mishna), quotes a teshuva by the Rashba which says that the Rambam had a change of mind on the matter, and the position presented in Mishneh Torah is the Rambam's later and final view. Halacha l'ma'aseh, Rav Yosef Karo, both in his commentary on the Tur (Beis Yosef) and on the Rambam, quotes his rebbe, Mahari Bei Rav, saying that l'chatchila one should only recite a beracha when leining from kosher sefer Torah. But b'dieved, e.g. if in the middle of the kriah one finds a mistake in the sefer Torah, one can rely on the original opinion of the Rambam and not reread the first part of the leining (from the second, kosher, sefer Torah which they would use in order to complete the sedra.)
Unlike Megillas Esther, where the halacha states that one can read from a Megillah which is missing a bit here and there as long as most of the text is intact, the Rambam's view regarding kashrus of a sefer Torah is that even if only one letter is missing, the sefer Torah is passul. Based on this position of the Rambam, the poskim have raised an issue regarding the kashrus of all sifrei Torah in the world because of one word in Parshas Ki Teitsei. There is a question regarding the correct spelling of the word "petzu'ah daka": should the word "daka" end with a hey or with an alef? If l'chatchila we follow the position of the Rambam in Mishneh Torah, then when there is no kosher sefer Torah available we ought not lein with berachos, based on the principle that safek berchaos l'hakeil. However, this is obviously not the common practice!
The generally accepted answer seems to be the suggestion of the Minchas Chinuch, who writes that if a word is misspelled, even in only by one letter, then the sefer Torah is passul. (For example, if the name "Noach" was spelled with a final nun in the beginning or a chof at the end. Yiddish folklore speaks of misspelling "Noach" with seven mistakes!) However, even though the word was not written in the sefer Torah in accordance with the mesorah, as long as according to the dictionary there is no error in the spelling, it is true that the sefer should be corrected but we would not declare it to be a possul sefer Torah. The word "daka" in Aramaic should end with an alef, while in Hebrew it should end with a hey, so either way we spell it there is not an error in the spelling, rather there is only a question as to whether the sefer Torah in front of us corresponds to the mesorah. Since that alone is not sufficient to passul the sefer, we all recite berachos on the kriah from all the sifrei Torah; b'dieved they are all kosher regardless of which spelling would be in accordance with the mesorah!