When I worked in BOS (with Air Canada) back in the mid 90's, we operated out of Term E, gate 8 to be exact. We used to have arrivals from YYZ/YUL/YHZ and YOW. At that time, YOW/YHZ didn't have US preclearance, so those arrivals had to go thru immigration upon arrival. The way it was set up was that when the psgrs deplaned, the precleared psgrs would make a right turn after walking up the steps, where we would open a door to allow them directly into the terminal. The non-precleared psgrs would make a left turn and proceed to the immigrations area. Occasionally we would make a mistake and the precleared psgrs would make a left turn. Immigrations would give us dirty looks,, but that was about it. More disastrous would be if the non precleared psgrs made that right turn into the terminal. We had to frantically get them turned around and back into immigrations.
Another occasional issue would be our Sydney NS-Yarmouth-Boston flight (back when Yarmouth had commercial service). Sometimes. due to fog, the flight had to overshoot Yarmouth and fly into Boston. All the psgrs would need to clear US immigrations, including the YHZ-Yarmouth ones. Those psgrs obviously didn't have passports. US immigrations was fine with that, usually just asked for any form of ID, Once out of customs, those psgrs would wait for the return Boston-Yarmouth-Sydney flight. By that time, the fog usually had burned away, and we got those psgrs into Yarmouth.