A long time ago when I was a student at Dalhousie, one of my professors brought in a guest lecturer for one of his classes. He was a graduate student and he was Black. He gave a great lecture and he acted very friendly and cool with the students.
A week or so later I had to go see my professor about something, so I went to his office. When I went in to his building the professor wasn’t there, but the graduate student was. I smiled when I saw him and said “hey, what’s up?” as though I was speaking to a friend. He gave me a cold stare and said “can I help you.” I mumbled I was looking for my professor but I’d come back later.
He made me feel a bit stupid, but good on him. I thought about it after and realized I probably wouldn’t have talked to him like that if he hadn’t been Black. When I talked to other lecturers I was more polite without even really thinking about it.