Off-By-One Error
Programming mistake that occurs when a programmer incorrectly references or iterates through an array or loop, resulting in the code either skipping over or repeating one element.
Origin
The error has been a recognized hazard since the earliest days of computing, when loop constructs and array indexing required exact boundary conditions. Edsger Dijkstra addressed the underlying issue in his 1982 note "Why Numbering Should Start at Zero," arguing that zero-based indexing eliminates an entire class of boundary errors. The informal term crystallized in C and Unix culture through the 1970s and 1980s, as languages that exposed direct memory indexing made the precise distinction between < and ≤ a constant source of bugs — and a rite of passage for every programmer learning boundary conditions.