I once arrived in Vancouver on an international flight from Europe and somehow they messed up the "routing" of passengers and everyone just walked out to the baggage carousels for domestic flights without going through customs at all! After a few minutes the employees realized what happened and they just brought over a few custom inspectors to the "exits" (it was really just a low wall with plants separating the carousels and the doors to the outside) and checked passports of people leaving, but a number had already left and they didn't even have a computer or anything, they just manually checked the passports. It's possible they only let Canadian through this way and directed foreign passports back to a real customs area, I don't exactly remember. It was pretty bizarre and only about 10 years ago now. Mistakes do happen!