Originally Posted by MSPeconomist

Often an airline attempts to "handle" the problems caused by a COS by making the encroached upon customer move to a different and often less desirable seat, for example Y+ to Y- or much further back. Another possibility is to make a parent hold a lap kid that would otherwise occupy a seat and use the kid's deat for the encroached upon passenger. If no seats are available in the same (or lower) cabin, usually the encroached upon customer is told to either accept the encroachment or leave the flight, (probably) to be (costlessly one would hope) rebooked onto the next available flight. If no space is available that day, the victim can pay for his/her own hotel room, meals, etc.