The regulation requiring passengers use the lav in the appropriate cabin can be found written on the side of the magic machine that allows the FA to see that there are still cell phones not in Airplane Mode.

And MSPeconomist, I've been advocating the hungry croc moat between Y and F for years. Along with the trap door leading to a similar moat that can open around the boarding area of the gate to clear gate lice. And the cone of silence in the compartment above, next to the air masks, that can drop on a chatty seatmate. And the seat that swallows the passenger who insists on opening the blinds in the middle of a flight to Asia despite the baby that cries non-stop when that's done.