Swap the photo they just picked, and most people won't notice

Researchers asked people to choose which of two faces they found more attractive, then handed back the opposite photo using a card trick. Most participants didn't notice the switch — and happily explained in detail why they'd 'chosen' a face they had actually rejected.

— Petter Johansson, Lars Hall, Sverker Sikström & Andreas Olsson, Failure to Detect Mismatches Between Intention and Outcome in a Simple Decision Task — Science, 2005