Off-by-one Error: N. Exceedingly Common Error Induced In Many Ways, Such As By Starting At

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:off-by-one error: n. Exceedingly common error induced in many
ways, such as by starting at 0 when you should have started at 1 or
vice-versa, or by writing `< N' instead of `<= N' or
vice-versa. Also applied to giving something to the person next to
the one who should have gotten it. Often confounded with
{fencepost error}, which is properly a particular subtype of it.
-- The AI Hackers Dictionary