the best-laid schemes go astray

Said when things that are well prepared for or seem certain end poorly or differently from how one intends. It is an abbreviated version of the line, "The best-laid schemes o' mice an' men gang aft a-gley" (go astray), from Robert Burns' poem "To a Mouse," which itself is a play on the proverb "the best-laid plans of mice and men oft go astray." I always thought our marriage was stable and that we'd be together forever. I guess it's true what they say, though, even the best-laid schemes go astray. A: "I've been working on this project for six months, and now, right before it's due, they tell me they want something completely different." B: "That's rough. Even the best-laid schemes go astray, I suppose."