Drive-by Friendship
The act of suddenly and quickly encountering an old acquaintance or friend (you may or may not remember), who you have not spoken to in a long time. However, the person greets you and speaks to you as if you have been close friends the entire time before leaving in a hurry.
Person on the street: "Hey man! How's it going? How are your classes? Are you still seeing that girl? We should hang out more. Give me a text or call me. Seriously, let's hang out later! Bye!"

You: "What the...who was that? Sheesh, I hate drive-by friendship!"