off-by-one error: n. Exceedingly common error induced in many ways,

:off-by-one error: n. 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 AI Hackers Dictionary

Related:

• off-by-one error n. [common] Exceedingly common error induced in many ways,
  such as by starting at 0 when you should have started...
• fencepost error n. 1. [common] A problem with the discrete equivalent of a boundary condition,
  often exhibited in programs by iterative loops....
• fencepost error: n. 1. A problem with the discrete equivalent of a boundary condition,
  often exhibited in programs by iterative loops....
• obi-wan error: /oh'bee-won` er'*r/ [RPI, from `off-by-one' and the Obi-Wan Kenobi character in "Star Wars"] n.
  A loop of some sort in which the index is off by...
• obi-wan error /oh'bee-won` er'*r/ n. [RPI, from `off-by-one' and the Obi-Wan Kenobi character in "Star Wars"] A loop of some sort in which the index is off by 1.
  Common when the index should have started from 0...
• zeroth: /zee'rohth/ adj. First. Among software designers,
  comes from C's and LISP's 0-based indexing of arrays...
• slop: n. 1. A one-sided {fudge factor}, that is, an allowance for error but in only one of two directions.
  For example, if you need a piece of wire 10 feet...
• check: n. A hardware-detected error condition, most commonly used to refer to actual hardware failures rather than software-induced traps.
  E.g., a `parity check' is the result of a hardware...
• banana problem: n. [from the story of the little girl who said "I know how to spell `banana',
  but I don't know when to stop"]. Not knowing where...