Borderline Case
/ˈbȯr-dər-ˌlīn kās/
noun — "the input that lives right on the edge of what a system expects."
A Borderline Case is a situation, input, condition, or value that falls at or near the limits of a system's expected behavior. It exists on the boundary between normal operation and exceptional behavior, often exposing assumptions, hidden bugs, or design weaknesses that may not appear during ordinary use.