I've heard a black passenger being questioned (by another passenger) about his standing in a queue of first- and business-class passengers for boarding. I don't know the reason for the mini-interrogation, of course, but it seemed fairly obvious to me at the time that it was probably because the passenger was the only non-white passenger in the line.

But I've also overheard a white man say to his white wife at an expensive resort hotel in California, when a black couple walked past, "They should stay with their own people." I couldn't believe I'd heard it.