last straw

the last straw/the final straw (the last in a series of bad events, etc. that makes it impossible for you to accept a situation any longer ) — последняя капля

Example: Bill had a bad day in school yesterday. He lost his knife on the way home, then he fell down, and when he broke a shoe lace, that was the last straw and he began to cry. (AmId) — ... но когда еще и шнурок порвался, он не выдержал и заплакал.

See also
[straw that breaks the camel's back]
[another nail in the coffin]