Off-by-one error

An off-by-one error is an exceedingly common computer bug 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.

The term "Obi-Wan error" is a sometimes used as a humorous synonym. The name is derived from the similarity between the phrase "off by one" and the name of the Star Wars character Obi-Wan Kenobi.