One year after your child is born you discover there was a mix-up at the hospital.
Three years after your child is born you discover there was a mix-up at the hospital.
Context: A real life case in Japan (although in that case they didn't find out until he was 60)
and also the discussion here on whether or not we should shuffle children at birth (although there seems to be under DDOS at the moment, so it may not respond quickly).
FWIW, Julie didn't like my simple solution that both sets of parents move in together and raise all the kids together. Nobody ever likes my ideas :-> (She just added that this would be fine if we could buy a mansion and have a wing each.)