Has happened a couple of times, the more interesting encounters being :-
1. Had met (let's call him) X exactly once. At work, X showed up one day as the new employee. Deer in headlights look on both faces. Quite amusing now to think back, but was terrified then. Needless to say, never met X again.
2. Had met Y at least a couple of times. At wedding reception of best friend, Y showed up as the new boyfriend of the bride's best friend. The fact of being acquainted already became apparent to others. Deer in the headlights again. But Y was unfazed, spun a yarn about the circumstances of the pre-acquaintance at the car dealership, place of Y's day job ostensibly.