OK, a simple one from a simple mind:
the police get an anonimous tip off that the murderer they are after is
at a specific house. They raid the house and find 4 people playing
poker - a chef, a surgeon, a fireman and a polititian. They knew
nothing about the man they wanted except that he would be in the
house, but they knew instantly who to arrest. How?
(and it wasn't because the murderer had used poisoned cup cakes
that were otherwise very tasty..)