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