The last straw that broke the camel’s back
A minor action that has a huge impact because of the cumulative effect
e.g. Last night I had a bad night’s sleep, my car battery was flat this morning, then the bus was late. When my work stapler was out of staples, I threw it across the room! It was the last straw that broke the camel’s back.
Sometimes, people just say:It was the last/final straw