One day on weekend cross cover a few years ago, I was rounding on an elderly trans-gender patient. Male to female. I went into the room to meet the patient, and if the resident hadn't told me she was transgender, I would have definitely thought it was a man. She had a deep voice, a stubble, and only slightly longish hair.
Anyway, as I was leaving the room, I said, "It was nice meeting you, sir." Then I quickly added, "Er, ma'am."
I was SO embarrassed. What's annoying is that I usually didn't even say "sir." I don't know why I felt compelled to add it with the one transgender patient I've ever had.
On Monday, I told the primary resident about it, and said I felt really bad. She said it happens all the time, which only made me feel slightly less guilty.