That happened to me once way back when I was 19 and had lost my first significant amount of weight. While I was at the grocery store I ran into one of the girls I used to sit with at the high school lunch table, so I called her out by name to say hi. She glanced me up and down, gave me a really dirty look, and said with a snarky tone, "Sorry, but I don't know you."
It was all kinds of awesome.