Hey! How are you?

You're walking down the hallway and you see someone you don't recognize. They smile... but are they smiling at you? You evaluate the eye contact, and it seems like they're looking at you as well. So as you get close enough to make it a reasonable greeting, you smile at them and at that precise moment... you realize they are smiling at a person behind you. 

In one word: awkward.

Some people may choose more than one word like humiliating, embarrassing, uncomfortable, etc.

Quickly, your mind reels as you try to think of a way to honorably duck out of the situation, and make it look like you weren't smiling at them, you were... um... suddenly, you have a stroke of genius. Of course! You were smiling at someone who was behind them. You shift your gaze so that you are no longer looking at the initial smiler, but you are looking behind and over their shoulder, smiling and waving. "Hey! How are you?" You quickly rush toward the corner and hold out your arms like you are getting ready to hug someone.

But, you failed to think ahead about the people around the corner. You come around the corner with your arms spread out wide, ready to hug... nobody. They think you're crazy, but at least you got out of the awkward situation with that one person who you thought was smiling at you.