"It was the last straw that broke the camel's back."
There is a limit to everything. We can load the camel with lots of straw, but finally it will be too much and the camel's back will break. And it is only a single straw that breaks its back - the last straw. This can be applied to many things in life. People often say "That's the last straw!" when they will not accept any more of something.