Ask HN: What's the biggest problem you ever prevented?

There was a recent thread [1] on a paper titled " Nobody ever gets credit for fixing problems that never happened".

I've seen good collections of postmortems before [2], but prevent problems are severely underreported, because why share your almost-failure if you aren't forced to (and other issues, like people not believing it was a big deal because it ended up not happening [3]).

So this got me wondering: What's the biggest problem you prevented from becoming a big problem?

1. https://news.ycombinator.com/item?id=48498385 2. https://github.com/danluu/post-mortems 3. https://en.wikipedia.org/wiki/Preparedness_paradox