Halsey didn't PLAN on showing the world her underwear at an amfAR Gala in NYC -- but when a dude stepped on her dress, pulling it down ... she was exposed like Trump's scalp in a wind tunnel.
The singer was rocking a high-cut black dress to the event when someone who appeared to be working the event tripped out and put his foot down on her Georges Hobeika gown.
Don't worry, Halsey was wearing undies, so it wasn't exactly a full Britney -- but still sucks.
Halsey composed herself, waved to the crowd and walked it -- and snatched her train back from under the shoe.
Bad at love. Good at recovery.