Originally Posted by
findark
I would assume the situation went something like GA offloads the toddler and hands a BP to the standby pax, assuming there is an empty seat on the plane. Standby pax boards, and confusion ensures. FA runs up (and is in a hurry) and mistakenly assumes that the mother booked a lap infant and misunderstood whether that entitled them to an extra seat.
It's not the most plausible story, but I could see it happening if the mother didn't communicate loudly and/or clearly that her son had a BP with a seat assignment and was entitled to the seat. Lapses in communication like that are far from unheard of. With this level of confusion, I doubt the FA checks the manifest carefully.
Seems pretty plausible to me. Plenty of people in the world absolutely terrified of making a scene (and even more likely terrified of doing so on a United airplane) and mom wasn't assertive enough about having a separate boarding pass for her kid to overcome the miscommunication with the FA.
That said, it also sounds like the FA may have bullied the passenger a bit by saying that PAX would lose their ticket if they didn't stay on that flight. And also the FA's fault for not recognizing a situation that didn't make sense.
Overall United is pretty much entirely at fault for this one.