So I work with a guy that I always see but never really talk to. For some reason, I had come to understand his name was "Mark."
After about 8 months of greeting him in passing, we have the following conversation:
Me: "Hi Mark!"
Mark: "Why do you always call me Mark?"
Me: "That's not your name??"
Not Mark: "No."
Me: "Why didn't you SAY so?!?"