the straw that breaks the donkey's back
A seemingly small or inconsequential issue, problem, or burden that proves to be the final catalyst in causing an overworked or overburdened person, system, organization, etc., to fail, give up, or collapse. (A less common variant of "the straw that breaks the camel's back.") I'm already fed up with your lazy, selfish behavior, Jim, but if you can't be bothered to come with me to my own mother's funeral, that will be the straw that breaks the donkey's back! With governmental resources already strained to the breaking point, any sort of environmental disaster would be the straw that breaks the donkey's back.