Parents move into a rent-controlled apartment in San Francisco and pay $1,495 a month. They've got (inter alia) a 13-year old child. Everyone lives together in harmony.
Years go by. The rent can legally only go up a little bit, so it's now $1,680 a month. Way below the market rent, which is double that.
The parents are older. So is the 13-year old. State law says that you can't raise the rent to market rates until the original "the original occupant or occupants who took possession of the the dwelling or unit . . . no longer permanently reside there." The only people on the lease are Parents.
Eventually, Parents (who are now older) move out, but Child -- who's now 23 -- stays.
Can Landlord raise the rent?
The Court of Appeal says: No.
Child was an original (lawful) occupant. Even though the lease was with Parents, Child can stay in the rent-controlled apartment, at rent-controlled rates, even after Parents leave.