I had a similar experience. Two gates side by side. One was C7, the other was C7C. I was waved onto the plane by the airline personnel who looked at my boarding pass. I was over the Gulf of Mexico when the pilot came on the p.a. system and announced our arrival time in Tampa. Two other passengers were on the wrong flight too. The airline took full responsibility for the mishap.
BelleAl can you tell me if that last post if that couple are the same person?