When a contractor comes into the office to sign for an ID badge and his mobile phone goes off, and you can tell him which starship enterprise the sound effect came from (red alert, Enterprise D).
Or the guy you're thinking of buying a bike off has one of the original (classic) Dr Who Cyberman costumes in his hallway and you can say that is a MK2.
Or you can give detailed information about famous scientists, their discoveries and the effects they had on modern life, but are at an absolute loss to try to identify any form of celebrity.