I don't think there is anything more awkward that when some starts talking to you like they know you, and you have no fucking idea who the hell they are.
While I was riding my bike past the horseback riding a woman stopped in the middle of leaving stopped in her truck and started talking to me, but I had no idea who she was. I was kinda behind the truck so couldn't really see her.
I did think of one possibility after she pulled away, but I'm not sure if that was her.
I truly hate when that happens because then when they are gone usually it clicks who they might be.