95% --> 100%
If you can touch-type at 100% of speaking speed, you can transcribe notes while people are talking. You won't miss anything and you don't have to make decisions: you just type what you're hearing, while you're hearing it.
If you can touch-type at 95% speed, your experience is qualitatively rather than quantitatively different. You keep missing things, and then you either skip certain phrases or you miss what the speaker is saying while you're catching up on typing what they said previously. It feels like treading water, rather than swimming.
Similarly, I'm told by the Getting Things Done crew that if you set up an organizational system where you're 100% sure that anything you need to do is definitely in your to-do system, you can attain a kind of inner bliss and free up the 10% of your brain that is constantly asking itself: "wait, am I forgetting something?"
In some sense, even being 99% organized is experientially closer to 95 than to 100 – you still have to run that separate, internal am i forgetting something? loop, you'll just less-often come up with something you've forgotten.
If you have weeds in your garden and you eliminate 95% of them, the weeds will grow back. If you eliminate 100% (and so do your neighbours, etc) you've saved yourself from ever dealing with them again.
Alas, infectious diseases have a similar dynamic: the benefits of globally eradicating a disease are very meaningfully larger than the benefits of getting even 99.999% close.
If you have a hole in your bucket, the water's gonna come out. Even if it's a small hole, over enough time all of the water's gonna come out. You really want your bucket to be 100% bucket.
By contrast, many other things in the world don't have this dynamic: doing them 95% is at least 95% as good as doing 100%, and frankly often that last 5% is just not worth the effort. Heck, famously, many things in the world are structured such that doing them 20%-ly gets you 80% of the benefits.
The examples above are in some sense the opposite – they're 20/80s, where doing 80% of the work gets you only 20% of the benefit. I wonder: what propert(ies) makes certain kinds of Things acquire much of their benefit only if you do them 100%, and far less benefit if you do even-slightly less?