Off-by-one error

An off-by-one error is 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.

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Article based on off-by-one error at FOLDOC, used with permission.