Learn English – “It’s good until it’s not” expression

English doesn't get much more precise than this...

Infinitesimal — immeasurably or incalculably minute.

It gets a bit mind-boggling when infinitesimal amounts are involved in, for example, homeopathic remedies that are so diluted there's only a very low probability of even a single molecule of the original substance being present. Which to me means an amount so small it's not actually there.

EDIT: I can't resist pointing out this answer itself now embodies an (almost) infinitesimal change. It was recently amended (not by me) to add a space before the first word. But you don't see it because of how the ELU display works (I only twigged by looking at the edit source).

Michael Crichton, in "Why Speculate?"—a speech he gave at the International Leadership Forum in La Jolla, California, on April 26, 2002—calls this phenomenon the "Murray Gell-Mann Amnesia effect":

Media carries with it a credibility that is totally undeserved. You have all experienced this, in what I call the Murray Gell-Mann Amnesia effect. (I call it by this name because I once discussed it with Murray Gell-Mann, and by dropping a famous name I imply greater importance to myself, and to the effect, than it would otherwise have.)

Briefly stated, the Gell-Mann Amnesia effect works as follows. You open the newspaper to an article on some subject you know well. In Murray’s case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the “wet streets cause rain” stories. Paper’s full of them.

In any case, you read with exasperation or amusement the multiple errors in a story—and then turn the page to national or international affairs, and read with renewed interest as if the rest of the newspaper was somehow more accurate about far-off Palestine than it was about the story you just read. You turn the page, and forget what you know.

That is the Gell-Mann Amnesia effect. I’d point out it does not operate in other arenas of life. In ordinary life, if somebody consistently exaggerates or lies to you, you soon discount everything they say. In court, there is the legal doctrine of falsus in uno, falsus in omnibus, which means untruthful in one part, untruthful in all. But when it comes to the media, we believe against evidence that it is probably worth our time to read other parts of the paper. When, in fact, it almost certainly isn’t. The only possible explanation for our behavior is amnesia.

The Internet being what it is—namely, devoid of the concept of irony—a number of sites have picked up on Crichton's "effect" and (I daresay) treated it with the profound respect that a carefully researched and peer-evaluated scientific theory deserves.