It's when someone whose intention is to wish on 11:11 accidentally wishes too early or too late. Is usually accompanied by harsh consequences, such as having wishes denied. Similar to an epic fail.
Person 1: "I just made a wish, I hope it comes true!"
Person 2: "Dude, it's 11:09. That's a false 11:11."
Person 1: "What the fuck does that mean?"
Person 2: "It means that your wish ain't coming true. So in your case, sorry, but you still have syphilis."
Person 1: "Stop reading my diary."