Right here:
Kohath, the son of Levi, had 4 sons (Ex. 6:18), who in turn had a total of 10 sons (Ex. 6:20-21; 1 Chr. 24:23-24), who in turn had 8,600 sons (Num. 3:28)! Surely there is a gap in this genealogy somewhere, they are called "clans" in Num. 3:28, which also implies some substantial history, as is indicated by the life of Levi being 137 years (Ex. 6:16), Kohath being 133 years (Ex. 6:18), and Amram living 137 years, yet if we say that they had all their sons at the very end of their life, we only get 407 total years, yet the time in Egypt spanned 430 years (Ex. 12:40), so there is not enough time in this genealogy to cover the time in Egypt, thus we probably have gaps in this genealogy. And presumably these Hebrew folks could add these numbers back then, and would have noticed such a discrepancy, if this was simply a numerical error.