When you're walking and someone in front of you decides to stop, or just move just enough in your way to throw you off. Like today. I was walking down the steps, and I'm a pretty fast walker. I make it to the end and this girl is heading towards the train when she decides to turn (God knows where cause there wasn't any train on the other side, and both would go the same way anyway) and I slam my jaw right on the top of her head. Luckily I didn't get busted open, but she turns around and gives me an "Ow!" WTF? It's your fault. Ok, maybe she's allowed to say ow, but it annoys me when people decide to change direction or stop at the exact time you're right behind them. I was like I'm sorry and continued my walk.