Your best supplier is your biggest capacity risk — not the shaky one you watch, but the excellent one you trust. This bites hardest when you single-source a proprietary or engineered component from a supplier running near full capacity: your start-of-production sits one of their other customers' orders away from slipping.
In other words, the supplier you are proudest to have qualified is usually the one with the least spare line time, because that is exactly what made them good. The risk never shows up as a line in your risk register. It shows up as a launch date that moves.
I watched this happen with a strategic engineered supplier: unique process, single source by design, the kind you are proud to have qualified. One of their large customers doubled a volume. Our SOP moved six weeks. No breach, no drama. The capacity had always been shared; we just were not in the room when it was allocated.
The load you never reserved
We held a unit-price agreement. They held a finite line. Price bought us parts; it bought us no claim on the hours. Capacity you do not reserve is capacity you do not have, and a framework that holds line time — even at a small premium — beats a cheap deal that queues you behind someone else's growth.
The ramp you never proved
The new program needed a rate this supplier had never sustained. Nameplate said they could. Their track record said otherwise. We committed volume to a ramp curve nobody had demonstrated, and the misses showed up as slipped dates, not as a risk flag. Stage the ramp: each step up earns the next.
The signal you never watched
Their on-time delivery to other customers slipped a month before ours did. Their hiring lagged the ramp. None of it reached our dashboard, because we were watching our own order, not their load. A supplier at full stretch shows it elsewhere first.
You cannot dual-source a proprietary die overnight, so the defense is not a second supplier — it is understanding. Know the load, hold the claim, watch the signal. A single source you understand is less risky than a dual source you don't.