The Hangover star completed a course at Pace University in New York, but skipped the opportunity to don the traditional hat and gown and collect his diploma in favour of filming a gay love scene in 2001 cult comedy Wet Hot American Summer.
He was left squirming in his seat when he came face-to-face with former school head-turned-interviewer James Lipton while taping an appearance on Inside the Actors Studio - and was quizzed over his whereabouts.
Placing the actor under the spotlight, Lipton asked, "Ten years ago as dean, I was handing out diplomas and I noticed you were missing from your graduation ceremony. Where were you?"
Cooper cheekily replied, "I was having sex with Michael Ian Black in a sports shed in Wet Hot American Summer."
The interview will air in the U.S. on Monday night (14Mar11).