"Excuse me, sir," I said, approaching her jokingly. "Are you new here?"
The woman turned around. It was not my coworker, M. It was some woman I'd never seen before, and she thought I had mistaken her for a man.
"I'm sorry," I said. "I thought you were someone else."
"I mean, some other girl. Sorry."
The woman gave me an annoyed look and returned to her phone call.
Lesson: If you don't want to be mistaken for a different person of your own gender but have the impression that you're being mistaken for a person of a different gender, then you should get a different damn haircut next time.