Only happened to me once. I was young with a young GF. She was giving me a handjob. I told her several times “don’t stop until I tell you”. Well in short order my cock shoots off a huge first stream of jizz. This was the first time she had seen a cock expelling cum. She was mesmerized. She stopped jerking my cock and my climax was ruined. I tried to revive it with my own hand but all I got was a thick dripple of cum.