You're focused on one person. Behind them, something else is happening that you didn't see until you reviewed the frame. The accidental photobomb - someone pulling a face, a dog jumping, a stranger making eye contact with your lens.

You can also cultivate this. Frame a clear background. Wait. Shoot when something interesting enters it, even if it's behind your main subject.

Watch for: the background element competing with the foreground subject for attention in a way that produces confusion rather than comedy. It needs to be funny or meaningful, not just busy.