Didn't happen to me but when I used to work the gate, I kept calling last call for a passenger for a flight they were going to miss, I had a gentleman sitting about 10 ft in front of me, looking right at me. I even asked them if they were going to that destination and they said no. Sure enough we closed up and left, and when I came back the passenger was sitting there with a bright red face realizing he was in fact supposed to be on that flight to that place.

Worst part was he held decent status with the airline, he was not an first timer in the flying department by any means.