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off-by-one

A bug where a count or index is wrong by exactly one — stopping one step too early or late.

Off-by-one errors come from miscounting boundaries: looping one iteration too many, reading one element past the end, or starting at 1 when you meant 0. They're famously common because the edges of ranges are easy to get subtly wrong. Small as they are, they cause crashes, corruption, and missed data.