Why checking the line up before admitting chemicals is important

 At 8:00 a.m. on March 13, 2019, an employee was working for a manufacturer of basic organic chemicals. He was opening a valve to permit the flow of hexamethylenediamine (HMD). The valve had been actuated to rinse and purge feed lines. It had been left open by the previous shift. The employee forgot to check valve positions before opening the HMD flow valve. He was sprayed with HMD, and he suffered second-degree burns to his groin. He was hospitalized. 

Source:osha.gov