I mean the spec and the live API behavior fall out of sync (often because implementation changes land first and the spec lags, or vice-versa). The first time we notice is when a real UI flow breaks and someone has to spelunk Devtools to see what the server actually returned (missing fields, nullability changes, new enum values, shape differences...)

So spec-diff tools like Vacuum help once you’re comparing two OpenAPI files, but my pain is earlierm catching “spec vs reality” from normal dev/staging usage (real accounts + data) and getting an actionable report (which operation, what mismatch, request id/response snippet) before it turns into a broken UI + an hour of debugging.