When I was working as a line cook in college, I was one of two white guys behind the line, with the rest of our BOH (Back Of House) crew being black. One day on the line we were all joking around like we always did. One of the black cooks, named Rose (he was an older guy, with coke-bottle glasses and strong, thickly calloused hands. He had a snaggle tooth and a big ol' pot belly), was regaling us with stories about how he has a veritable harem of women at his beck and call. He told us he was supporting like 5 different women, and all of them were entirely devoted to him.
I thought that he was being hyperbolic, and decided that I would poke fun. I was going to say something like "Rose, I can't believe for a second that even 1 woman would be after your ugly mug." But my midwestern-whiteness shone through, and before I could deliver that absolute blister of a line, I decided to use an exclamation so as to punctuate my lack of belief in his statement. I started with "Ooh boy," and didn't get to finish.
I was immediately accosted by Rose, his spatula gripped tightly, and he was mad as hell. The other line cooks were instantly aware of the situation and reacted to hold him back, as Rose was about to knock my befuddled ass into next week. Eventually they were able to calm him down, and explained to my dumbass that calling a black man "boy" was explicitly racist, and derogatory.
We were fine once he realized I had no idea it was racist, I just thought of it as an exclamation along the lines of "Boy howdy!" or something.
It was a very eye-opening moment.