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Someone turned off the the escalator in the UN building to mess with Trump as soon as he got on.
(v.redd.it)
There is usually 2 stop buttons, and then keyholes to re-start it (which you have to empty the staircase to do safely)
You can clearly see no one hit the stop button at the bottom.
Standardly when you lift the handle to press the emergency stop button you hear an buzzer like sound, much like a shot clock at a basketball game going off. If they didn't hear that noise, it likely wasn't a person who stopped it, but rather the escalator stopping because it thought something got stuck, or there was a malfunction.
Source: first job was a movie theater on the second floor of these things called malls that used to exist. Used to have to start/stop them and deal with kids lifting the lever to think about pressing the emergency stop, but the buzz noise would usually sway them from doing so