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.