Had to read the fine article to figure out what "NTO" was.
Over the course of ground testing, a check valve separating the pressurization system and oxidizer leaked what SpaceX described as a slug of nitrogen tetroxide oxidizer (NTO) into the helium pressurization lines."
4 posted on 07/16/2019 6:08:04 AM PDT by Yo-Yo
( is the /sarc tag really necessary?)
Over the course of ground testing, a check valve separating the pressurization system and oxidizer leaked what SpaceX described as a slug of nitrogen tetroxide oxidizer (NTO) into the helium pressurization lines."