The full phrase is
"the best laid plans o' mice an' men gang aft agley"
This was written by an ancient Scots Poet, Robert Burns, who wrote several poems in a strange language, including "To a mouse".
"Gang aft agley" means "often get fucked up"
"best laid plans" is an acknowledgement that something has just got fucked up for no apparent reason, or because something inevitable (government, parents, teenage kid, fate, whatever) has acted against it.
Billy: Damn. Eight point buck just standing there. Gun aimed, ready to take the shot, I was so damn sure that I could take a piss without letting go of the gun. But I peed all down the front of my pants, the gun went off before I was ready, that scared the buck who ran right at me and kicked my ribs in.
Bobbette: you know what they say, honey, best laid plans and stuff. Now turn over and let me wipe your butt. Eeewwww!
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