Word or Phrase for the many minor things adding up to something major and not noticing until it's too late? [duplicate]

That would be Boiling Frog Syndrome ^_Warning!1. As Wikipedia explains...

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 straw that broke the camel's back

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.