This reminds me of a weird thing that happened:

So I have a younger cousin, much younger – as in a 20-year age difference, give or take. I had seen her a bunch of times at family events and the like, always looking at her and saying hi to her, but all those times she was a little kid and not quite at the "meaningfully aware of your surroundings" stage of mental development.

So last Xmas I go to another family thing, and there's my cousin, who I haven't seen in months. And of course she's all grown up and much more alert, so go over to say hello, to which she responds "uh, who are you?"

Tl;Dr: I had to introduce myself to the cousin I've known for three years because she was too young all that time to remember me.