Today I boarded the wrong flight

Yes, I mean what you understood. I walked through the gate and got into the plane, but it wasn't the right one. Fortunately, another passenger had the same seat; otherwise, I would have flown in blissful ignorance, while my baggage was being unloaded from the right flight, searched and probably destroyed.
It was a short hop, Dublin to London City, traveling by Air France. I went to the gate and found an Air France flight leaving at the same hour and same plane model. I gave my boarding pass to the stewardess at the gate, and went through. I realized that it was a different gate, but heck, same company, same destination, same plane; gates change, shit happens.
I was about to sit down, and I found this other guy at my seat. We checked both our stubs, and both had the same seat. The plane was empty, so I asked the stewardess if I could change my seat (come on, seats might get double assigned too). At first, the stewardess looked baffled at the other guy's stub, since she was not used to see that kind of stub, but after a brief check, she started looking at what codes that flight had (at least two codes). Suddenly they realize I was in the wrong plane. I disembarked and everything went well; I just wonder what might have happened if the seat was empty (and if the destination weren't the same)
PS: part of this post was written on board the right plane