If I were a member of U2, here's what I'd do to troll my fans. In the middle of a show, in an insignificant place (for U2) like Omaha or Copenhagen, just play "Street Missions" or "The Fool" randomly. Don't announce it beforehand, don't say anything after it's over. Just one heartfelt performance... and then never play it again, and never mention it. It would drive everybody absolutely crazy!!!
Any of those old tunes will work. Cartoon World, False Prophet, Magic Carpet, etc.