Until we got our dry pump I had it tattooed on the door of every oven: “YOU MUST CLOSE THE VALVE BEFORE TURNING OFF THE PUMP”.
Even with a pressure switch controlling a solenoid to protect against it, the idea of 2 quarts of type 19 coating our slabs still generates substantial “pucker factor”.
I once saw a $1.2M gold plating on the inside of a massive Alcatel deep vacuum chamber get misted during roughing for a leak test because they didn’t install an anti-suckback valve. Ended up not hurting anything but holy fuck did everyone in the shop for it fear for their lives/jobs. I was happy to just be the kid that installed the o-rings that day.