On some RJ's, there is an extra mask on one side of the plane only. We didn't just show up, it was in the record, but no one caught it until we were boarding.

Nice people "had" to move to the other side and had to be separated. 1x2 seating. They didn't seem to mind once they knew why.

I am sure that had someone caught it before, we wouldn't have had to inconvenience anyone.