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off-by-one error: n.

[common] 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.