On each of the eight days of Hanukah, we read a special reading from the Torah, from the section of the "Nesi’im" in Parashat Naso (Bamidbar, chapter 7). If a congregation mistakenly read from the Torah that week’s Parasha, instead of the special Hanukah reading, and they realized their mistake only after completing the reading, they do not need to then read the Hanukah reading. This is the ruling of Hacham Ovadia Yosef, in his work Hazon Ovadia (Hanukah, p. 239), where he writes, "En Mashgihin Ba’Hanukah" – we are not as insistent with regard to the Hanukah Torah reading as we are regarding other readings.
On Rosh Hodesh Tebet (which always occurs during Hanukah), we remove two Torah scrolls from the ark (unless this occurs on Shabbat, in which case we remove three scrolls). From the first Torah, we read the section of Rosh Hodesh, and three Aliyot are called for this reading. For the fourth Aliya, we read from the second Torah the section for Hanukah. If a congregation mistakenly read four Aliyot in the section of Rosh Hodesh from the first Torah, then they must call a fifth person for the Hanukah reading from the second Torah. It is considered offensive to a Torah scroll when it is removed from the ark but not used for reading. Therefore, if all four Aliyot were mistakenly read from the first Torah, an additional Aliya must be made to facilitate a reading from the second Torah.
Summary: If on Hanukah a congregation mistakenly read that week’s Torah portion instead of the special Hanukah reading, they have nevertheless fulfilled their obligation and do not need to read the Hanukah reading. If on Rosh Hodesh Tebet a congregation mistakenly read all four Aliyot from the first Torah scroll, they must make a fifth Aliya for the Hanukah reading from the second scroll.
