"A system is never the sum of its parts; it is the product of their interactions.
Ackoff’s line is a check on the comforting habit of component thinking. You can make each part look healthy and still build a system that fails at the seams. Local health can hide global failure.
In engineering, the seams are timeouts, retries, caches, and handoffs between teams. A small change to improve reliability in one service can trigger a retry storm in another; a policy that improves throughput can shift work onto the team least able to absorb it.
Before you ship the change, name the interaction you are relying on. Write what must stay true across boundaries and the first signal that it is no longer true, so the system—not your optimism—gets the final vote. Make it specific enough that someone can disagree.