No plan of operations extends with any certainty beyond the first contact with the main hostile force.
~ Helmuth von Moltke the Elder
Everybody has a plan until they get punched in the mouth.
~ Mike Tyson
Scottish poet Robert Burns aptly penned the famous phrase: "The best laid schemes o' mice an' men/Gang aft a-gley." (commonly adapted as "The best laid plans of mice and men often go awry.")