Status: IN
`most_frequent_number_following_key_in_an_array` raises `IndexError` if `key` only appears at the terminal position — the `range(len-1)` bound correctly skips it, but if no earlier occurrence exists the `Counter` is empty and `most_common(1)[0][0]` fails.
Source: entries/2026/06/06/most-frequent-number-following-key-in-an-array-solution.md