Had a slightly scary situation on AC 725 last Friday. The plane was completely full. One passenger came to his seat, only to find another passenger was already sitting in the seat. Upon further inspection, it was noticed that the passenger sitting in the seat was on the wrong plane!

How is that possible in these days? I know there is just a rapid bp-check at the gate, and apparently nobody collect bp:s, there's no electronical scanning etc. But it doesn't give very much confidence! I consider it a serious security breach that I doubt would happen on European airlines, where all boarding passes are scanned.

Is this common in US/Canada?