If this a simple mistake of flying on the wrong date, it means that not only did the gate agent not pick up on the passenger’s mistake, but that the TSA security didn’t check the date on the ticket/boarding pass. Three separate mistakes not picked up until hours into the flight. If there was also a change in seat assignment made, it could be 4 mistakes, plus what looks to a 5th error in the boarding scanner computer system. Yikes.
I’m not sure what the issue is, but I could see that it was a failure for UA to share info with ANA on a late rebooking or something like that. The computer would catch it and things would work fine, but the printed manifest may not have been up to date.....OR much weirder things are going on - like someone else booked the ticket and then switched it with someone else.
I think the more likely scenario, and the only reason I can think that they didn’t continue to their destination is that this person wasn’t allowed to go to Japan by either Japan or the US. ie flight risk, under investigation with a hold, material witness...any number of reasons. Only thing I can figure that would cause them to turn the plane around and inconvenience the other 150+ passengers.