Akademik

Гладко было на бумаге, да забыли про овраги
Despite careful planning, things often go wrong in their final accomplishment
Cf: The best-laid plans of mice and men often go astray (Am.). The best-laid schemes of mice and men gang aft agley (Am., Br.). It sounded (sounds) right on paper (Br.)

Русско-английский словарь пословиц и поговорок. . 2013.