When you see a person you know on the street, and they see/recognize you, but pretend they didn't and look away in order to avoid talking to you. Usually results in an unnatural looking facial expression.
Laura: Dude, I saw Russel today, but he totally blanked me.
Meag: Really? I thought you guys were friends?
Laura: Yeah, me too. I guess he just didn't want to talk to me.