The Jargon File (version 4.4.7, 29 Dec 2003):

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.


The Free On-line Dictionary of Computing (30 December 2018):

off-by-one error

    (Or "Obi-Wan error") An exceedingly common error
   induced in many ways, such as by starting at zero when you
   should have started at one or vice-versa, or by writing "< N"
   instead of "<= N" or vice-versa.  Often confounded with
   fencepost error, which is properly a particular subtype of
   it.

   The term zeroth corrects the linguistic off-by-one error of,
   e.g., referring to the "1st" element of an array whose indexes
   start from zero.

   [Jargon File]

   (1998-09-21)