Source : Free On-Line Dictionary of Computing

fencepost error
     
        1. (Rarely "lamp-post error") A problem with the discrete
        equivalent of a {boundary condition}, often exhibited in
        programs by iterative loops.  From the following problem: "If
        you build a fence 100 feet long with posts 10 feet apart, how
        many posts do you need?"  (Either 9 or 11 is a better answer
        than the obvious 10).
     
        For example, suppose you have a long list or array of items,
        and want to process items m through n; how many items are
        there?  The obvious answer is n - m, but that is off by one;
        the right answer is n - m + 1.  The "obvious" formula exhibits
        a fencepost error.
     
        See also {zeroth} and note that not all {off-by-one errors}
        are fencepost errors.  The game of Musical Chairs involves a
        catastrophic off-by-one error where N people try to sit in N -
        1 chairs, but it's not a fencepost error.  Fencepost errors
        come from counting things rather than the spaces between them,
        or vice versa, or by neglecting to consider whether one should
        count one or both ends of a row.
     
        2. (Rare) An error induced by unexpected regularities in input
        values, which can (for instance) completely thwart a
        theoretically efficient {binary tree} or {hash coding}
        implementation.  The error here involves the difference
        between expected and worst case behaviours of an {algorithm}.
     
        [{Jargon File}]
     
        (1994-12-01)