I need a word, phrase, expression, metaphor etc for when lots of "insignificant" incidents or mistakes etc add up to make something major but by the time you realize it's major it's too late.
The boiling frog is an anecdote describing a frog slowly being boiled alive. The premise is that if a frog is put suddenly into boiling water, it will jump out, but if it is put in cold water which is then brought to a boil slowly, it will not perceive the danger and will be cooked to death. The story is often used as a metaphor for the inability or unwillingness of people to react to or be aware of threats that rise gradually.
1 I haven't actually followed that Youtube link. Don't blame me if it's too graphic!
The idiom the straw that broke the camel's back, alluding to the proverb "it is the last straw that breaks the camel's back", describes the seemingly minor or routine action which causes an unpredictably large and sudden reaction, because of the cumulative effect of small actions.
"Nickle-and-dimed" (to death / into poverty) is an idiom in which tiny costs or losses (nickles and dimes) add up to a large negative outcome (being broke, in debt, or bankrupt). It tends to be specifically about transactions -- you wouldn't say "the boxer nickle-and-timed his opponent."
By comparison, "the straw that broke the camel's back" and "boiling frog syndrome" (discussed in other answers) both emphasize that the minor changes (each extra straw, the water temperature rising) are unnoticed until the sudden negative event (broken back, death). The frog doesn't jump out of the water, the camel doesn't complain. By contrast, the cuts and the nickles emphasize that the changes aren't unnoticed, but they seem minor -- until they add up!
On the opposite extreme, "Nibbled to death by ducks" is to be subject to constant petty annoyances, according to The Dictionary of American Slang, Fourth Edition. In contrast to the above idioms, here I believe the joke is that these small events never add up to one big negative event -- the ducks don't ever actually kill you, they just nibble and nibble, so to be nibbled to death by ducks is never to be killed at all, only annoyed. Contrast again the "death of a thousand cuts" which was a real historical torture and execution practice.
The snowball effect is one good option. Usually something suffering from this effect is described as "snowballing out of control."
You could use "For want of a nail..."
For want of a nail the shoe was lost. For want of a shoe the horse was lost. For want of a horse the rider was lost. For want of a rider the message was lost. For want of a message the battle was lost. For want of a battle the kingdom was lost. And all for the want of a horseshoe nail.
Popularized by Ben Franklin, but much older than that.
I will give you two idioms to string together to express your idea.
*One thing led to another and the next she knew, things had gotten past the point of no return.
point of no return: the halfway point; the point at which it is too late to turn back. (Often with past.) The flight was past the point of no return, so we had to continue to our destination.
This may also be a case of Domino Effect.
domino effect NOUN
The effect of the domino theory.
‘Equally worrying is the fact that delays are causing a costly domino effect as contractors incur costs while they wait for other workers to complete tasks.’
‘Such selfish driving practices prevent other drivers from being able to park correctly, which sometimes creates a domino effect in a line of spaces.’
A non-idiomatic way of expressing the same conditions as "a frog on a hot plate" is ignoring the cumulative effect of the situation until its too late. A related adjective is accumulative, but that has more to do with intensifying or magnifying. See Quora for more info.
"The straw that broke the camel's back" doesn't work for this one. Comparing your situation to frogs in boiling milk will probably work, but it honestly depends on the context and you really don't want to use it for anything else other than comparison (in my opinion anyway, because it just sounds weird).
If you refer to a situation as "the boiling frog syndrome" then people will know what you're talking about, but it sounds strange compared to referring to a situation as "slowly spiralling out of control" while you were too busy to notice.