When a guy is watching porn and just in the moment of ejaculation the video switches to a shot of the male porn actor's face, or the male porn actor pulls out to change positions without any warning such as a slow dissolve. Or any undesired shot that pops up just when you're cumming. Further examples: shots of the guy's ass hole while camera is underneith a doggy-style position, any shot where you can see the guy's ass hole or it's too close to the camera, or any shot where the female leaves the screen. They spoil your apex and make you want to start over in many cases.
You're better off watching POV if you're going to jack off, or you run the risk of catching a cum-spoiler right in the middle of your nut