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.

[glossary]
[Reference(s) to this entry by made by: {fencepost error}{obi-wan error}{walk off the end of}]