To be the straw that broke the camel's back
Literally 'to be the drop that overflowed the glass'. The final small thing that causes a reaction after a long build-up of frustration.
Arriving late one more time was the straw that broke the camel's back.
—Why did you resign? —That comment was the straw that broke the camel's back.
She put up with a lot, but that was the straw that broke the camel's back.