Source : Free On-Line Dictionary of Computing

off-by-one 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.
     
        [{Jargon File}]
     
        (1998-09-21)