This happened to my wife and I on an AC flight in August. We were in LGA heading to YYZ and our flight was delayed close to the departure time for the next YYZ flight. They started boarding for our flight and when we got onto the plane, there was a man and a woman in our seats. They were supposed to be on the next YYZ flight, so same seats, but wrong flight #. Both the passengers and gate agents clearly weren't paying attention.