^ that happens to me too, this sorta friend was smiling so I smiled and waved and she was smiling at this guy behind me and totally didn't even see me.

I don't really initiate the waving or smiling unless I know for sure they are looking at me. So usually I come off as being a bitch cuz I didn't smile or wave back at you, but really, I didn't know you were waving or smiling at me to begin with.