Finding a mistake in a sefer

It's one of those dramatic shul moments. The reader is sailing along with the Torah leyning on Shabbat morning, when suddenly he stops mid-verse. He squints at the Torah scroll, frowns and calls over the rabbi.

The rabbi bends over to examine the scroll and signals that it should be rolled up, put back in the ark and another scroll brought out for the rest of the reading. A mistake has been found in the Sefer Torah!

Do we then have to go back and read again what was read from the invalid scroll. This turns on a disagreement between two medieval authorities Rambam and Rashba. Rambam opined that we do not; the mitzvah is to read from the Sefer Torah and we did that. Rashba disagreed believing that the mitzvah is to read from a kosher scroll and we did not; therefore we must start from the beginning from a valid Sefer Torah.

The halachah is that we do not need to reread what was already leyned, because we do not want to inconvenience the community.