Not pre-existing memory loss. That's a classic trope for good reason. It's when something happens to the characters, and then, mid story, they lose their memory of it.
So I'm reading along trying to remember who remembers what, and who doesn't, when I remember it all. Juggle, juggle, juggle. . .
Naturally I've got a story with characters forgetting something that happened in story. And now I suspect that I'm going to have to make them forget something else, too. . .