[HCIdivision17]

Oh gosh, having worked on a fairly large vacuum system, I can tell you that o-rings are monsters. Very minor errors in dimensions can mess up the seal, and temperature/humidity/wear/elasticity and all that can subtly mess with the dimensions in crazy nonlinear ways. You can simulate the ever loving garbage out of it and an imperceptible change in composition due to an undetectible mixing error when extruding the ring can cause a seal to slightly leak. Mayhem ensues. (And most likely any attempt to directly detect it will destroy the integrity of the o-ring or take so long to render the test useless, since there are usually hundreds of o-rings (or in the thousands - o-rings are all over the place).)

I'm not even talking about jackquesm's note about the failure mode, either. Just real insidious errors in manufacturing that can't be detected in any sort of reliable, sane way. Even the Challenger's o-ring wasn't guaranteed to fail, and indeed most didn't. In fact, most of that entire o-ring didn't fail.

I've seen some really freaky things amplify what are essentially chaotic edge cases. You can certainly figure them out, but you'd never get anything done for any level of affordability in time for any ship date if you didn't just calculate risk and go ahead.

TL;DR: risk is always there because the world's imperfect. At best you just tighten the statistical confidence, but that's super hard.