Here's what I've found out and will put into a story for Monday (so -- other news media lurkers, do NOT use this, it's embargoed). The flawed mirror was not on the spacecraft, but was used to project a test image INTO the spacecraft optics. At that point the spacecraft optics were adjusted -- either by moving the mirror or the detector plane -- until thay had perfect focus. Then the hardware was nailed down permanently. But the test stand mirror warped slightly because the test was conducted at the expected temperature of the actual encounter, out at 1.8 AU. No other instruments undergoing the same type of testing were affected that way -- it's a one off f__k-up.