My girlfriend says that a common case of motivated cognition is witnesses picking someone out of a lineup. They want to recognize the criminal, so given five faces they're very likely to pick one even if the real criminal's not there, whereas if people are leafing through a big book of mugshots they're less likely to make a false positive identification.

She suggests a prank-type exercise where there are two plants in the class. Plant A, who wears a hoodie and sunglasses, leaves to go to the bathroom, whereupon Plant B announces that they're pretty sure Plant A is actually $FAMOUS_ACTOR here incognito. Plant A pokes his head in, says he needs to go take a call, and leaves. See who manages to talk themselves into thinking that really is the celebrity.