the straw that broke 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 broke the camel's back.") I was already fed up with my husband's lazy, selfish ways, but it was his refusal to get off the couch and come with me to my mother's funeral that was the straw that broke the donkey's back! With governmental resources already strained to the breaking point, any sort of environmental disaster would likely be the straw that broke the donkey's back.