A BA 777 rolled back from the terminal at LHR just after the PAX had disembarked a few years ago, Capt also floored brake pedals with no effect so they declared a Mayday and started the engines which was the only way to arrest the roll. Can't remember how far it went.
We've been through stages of requiring chocks in place before any disembarkation, to not needing them so long as they are put in place before the crew leave the aircraft. As you can imagine the policy changed again.
Also not all airlines release the brakes once the chocks are place either.