A US-based psychologist, Kip Williams, was strolling through a park when a mis-thrown Frisbee caught him on the back of his head. Unhurt, he picked it up and threw it back to one of the players. They threw it back to him. Briefly he was involved in their game, before the Frisbee players stopped passing it to him and returned to their back-and-forth routine.
A relatively unremarkable social interaction, the scientist was nonetheless surprised at how hurt he felt at being excluded from the game and hurried off to his lab to ascertain what is happening in our brains when we feel left out.