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.