When the pax is seated in the exit row.
FA : Excuse me mam, I will just pop your bag in the overhead locker as your in an exit row.
PAX : Oh that is okay, I will just leave it here on the floor thank you.
I'll fess up to being the PAX end of that one on an LGW-AMS flight. In the states, its perfectly fine to have floor luggage underneath the seat in front in an exit row.