In my case, the KE people told me that the SQ people had made an error in SFO in allowing me to board with my itinerary. In fact, they passed the problem on to SQ and it was SQ that got me a hotel in Seoul for the night. KE, I think, cancelled my ticket and issued me a new ticket for the next day's flight at the same time as the original flight, so I eventually spent more than 24 hours in Seoul. (Technically, because I spent quite some time airside at ICN while the mess was being sorted out, my passport stamp shows me as having spent only about 23 hours in Korea, but the U.S. guys did not catch it when I landed in Guam.)

BTW, I am surprised about having to show the originating flight's BP at ICN. I no longer transit through ICN these days, but I used to transit through ICN all the time while flying SQ to SFO and do not recall ever having to show my arriving boarding pass. These days, I transit through HKG flying CX and have never been asked for my arriving boarding pass. They only ask for my onward boarding pass.