The story
From a Scots poem by Robert Burns, To a Mouse. In the original Scots, the lines are, “The best-laid schemes o’ mice an’ men / Gang aft agley”.
Etymology from Wiktionary, CC BY-SA 4.0.
the best laid plans of mice and men often go awry
Today it means: No matter how well a project is planned, accidents or misfortune can still occur. Full definition →