A proper screw-up, blunder, or mess you’ve made of something. You’ll hear it in the classic confession: I made a right cock-up of that, meaning it’s gone spectacularly pear-shaped and it’s on you. Roughly like balls-up, maybe a touch less rude depending on the company. Very British, very honest.
"Who ordered five hundred napkins instead of fifty? Yeah, that was my cock-up. Now the office looks like a wedding venue in Wetherspoons"