During the pre 9/11 era, I was travelling to the US for a (long) weekend almost every other month, often with my best friend. Then passport was not required but I always carried one. Since I usually took care of the paperwork for both of us, I also became a custodian of both passports, just taking them out when needed. Once, I was going through the US immigration at YUL with my friend's passport and he presented mine at the next wicket without being aware of that. However, both officers noticed, pointed that out and let us pass with a . Nowadays, we would probably be detained.