Micah’s error is that he selected a multiple that is not the least of the common multiples.
The least common multiple (LCM) of 8 and 12 is actually 24, not 12. Micah correctly listed the multiples but incorrectly identified 12 as the least common multiple when it is only a multiple of 12 and not a common multiple of both. The smallest common multiple of both 8 and 12 is indeed 24, which is the first number that appears in both lists.