For me the concept applies to someone you just hate but unfortunately probably doesn’t even remember you exist.

For example I had a shit boss at my last job and I’m coming up on a year since I left. I can’t forget that guy. He probably forgot I even existed a couple days after I quit. I don’t forget though how much of a dick he was once you saw his true colors and I know the other 2 guys who both quit before me don’t either. Those guys tried warning me to find a job before I inevitably did something to set the guy off. Unfortunately I got to see it first hand and it was just baffling he had remained in business.