Someone must have disabled the failsafe — there is a mechanical flywheel that is supposed to keep that from happening (I saw how it works on a Mythbusters episode).
Man, strange.
I was on one once that ran away upwards, not down. Stopped on 28th floor and quite a hike to take stairs to 5th floor where I had wanted to stop.
A friend that worked on elevators told the greatest risk is an elevator running away up, not down. Either way, scary.