Overly broad config hashing caused 15 units to invalidate from a 6-line edit

A software project used SHA-256 hashes of an entire shared configuration file to verify whether individual units were up to date. When a developer added just six lines describing one new component, all fifteen units were marked stale and required full re-runs, consuming nearly an hour of machine time. The root cause was a mismatch between what the hash actually guaranteed — that no byte in the file had changed — and what engineers assumed it meant: that only their unit's relevant inputs had changed. The fix involved redefining the pin to hash each unit's own configuration block plus genuinely shared sections, rather than the whole file. The episode highlights that hash granularity is a design contract, and a check that is technically correct but disproportionately costly can quietly erode engineering practices over time.