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."