The act of masturbatingin the back yard of the girl you are currently stalking and cumming inside of her fire hose. She doesn't have to see you and you don't have to be staring at her, or a picture of her, to be pulling a joker's card.
What you type into your web browser after you type in something sketchy into your web browser. You type it to make sure the FBI doesn't get you. I promise it was a joke.
A kid on a chatroom joked about blowing up a daycare. A week later, police storm his house and arrest him. When questioned why during interrogation, he said: "It was just a joke" and then he was sentenced to three years in prison.
Example #2
A drunk news anchor accidently says "nigger" during a leaked sex tape. When questioned by the public why did he say it, the anchor replied "it was just a joke" and was promptly fired from his job.