Originally Posted by
totti
Are you sure that you do not have to go through Canadian Customs on Intl => Intl. and US => Intl. connections?
There is a special transfer area for those cases (not new). Up until recently, unlike e.g. in YYZ, in YUL the CBSA agents in those booths had to essentially go through the full proceedure for entering Canada, because of the terminal design.
Now, they can limit themselves to the quick transit-style look at your passport that is already used in YYZ.