Swap the person mid-conversation — half the room won't notice

In a 1998 study, researchers had someone stop a pedestrian to ask for directions. Seconds later, a door carried between them by two "workers" briefly blocked the view — and a completely different person took over the conversation. Roughly half the pedestrians never noticed the swap, showing that change blindness holds even face-to-face, not just on a screen.

— Daniel J. Simons and Daniel T. Levin, Failure to Detect Changes to People During a Real-World Interaction — Psychonomic Bulletin & Review, 1998