On 17 November 2021, shortly after a Boeing 737-800 commenced initial descent into Patna from FL350, a cautionary alert indicating automatic pressurisation system failure was annunciated. When the initial actions of the prescribed non-normal procedure did not resolve the problem, the system outflow valve was fully opened and a rapid depressurisation followed. After this incorrect action, the relevant crew emergency procedures were then comprehensively not properly followed and it was further concluded that the Captain had temporarily lost consciousness after a delay in donning his oxygen mask. The context for the mismanaged response was identified as outflow valve in-service failure.