We pre-booked our seats months and months ago. We travelled home from Orlando recently, and received the seat assignments on our boarding passes.
As we were LINING UP to board, my husband was called to the podium and assigned a new seat. He had been moved arbitrarily to accommodate a family (not with small children, either). The fact that Air Canada was breaking up our family didn't matter one bit to the gate agent. She told him the flight was full and that he could complain when he landed in Toronto.
If they had asked, it would be another matter. But they didn't.
We have two trips planned south at the moment -- and they're both through Buffalo. Air Canada has done it again.