the best-laid plans go astray
Said when something ends poorly or differently than expected, despite preparations for success. It is an abbreviated version of the full 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 plans 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 plans go astray, I suppose."