As a white Canadian male, I don't think I've ever been on the receiving end of racism, and thankfully I don't think I've seen it too often since I've generally lived in fairly multicultural neighbourhoods and gone to pretty ethnically diverse schools. However, that's meant the few times I have experienced it, they really stand out in my memory:
1) The summer where racists were trying to make the "Ground Zero Mosque" a thing, I was at a bank getting some money out. The cashier (a white woman in her 50s) went to the back, and when she returned she saw the TV behind her was playing a story about it.
Her, shaking her head: "Such a shame."
Me: "...what do you mean by that?"
Her: "Those Muslims, they're taking over everywhere."
I don't think I've ever yelled at complete strangers as much as I did after that. I ripped into her for being a racist idiot, and told her that I didn't want her touching my money with her bigoted hands. Then I walked over to the customer service desk, and filed a complaint about her with the branch. Then I went home and called my bank to file another official complaint, this time with the corporate headquarters. I never heard back from the bank branch, but the HQ called me to let me know she'd been ordered to take sensitivity training, which is better than nothing, I guess.
2) At the height of the Mohamed cartoons controversy (so, like, 2006?), I was visiting a friend's apartment. Her roommate and I got into a heated debate about it, which ended with the roommate yelling at me that it was all a Jewish conspiracy, that the Jews controlled everything, etc. I left, and the next evening I met up with my friend to walk somewhere. The walk lasted, like, a minute, because our conversation went something like:
Her: "Sorry, things got out of hand yesterday..."
Me: "Yeah, that was pretty crazy."
Her: "...but she's right, you know. The Jews really do control everything."
That was the last time I ever spoke with her. It was really weird, and pretty disappointing, because the friend (a black woman) was on all sorts of multicultural committees and groups, and otherwise had never said or done anything to indicate she believed in racist conspiracy theories.
And just to counter the bad stories, one that turned out well: in 2003 I was visiting London, and a little lost, so I walked for awhile until I got tired and sat down on a bench. I put my bag down next to me, and when someone went to sit down next to me, I pulled the bag off the seat and onto my lap so that I wasn't taking up two spots on the bench...only for the person to assume I did it because he was black. He yelled at me for being racist as I tried to explain myself, but he wasn't having any of it. Finally, he ended by shouting, "People are such cunts!"
Me: "Well...yeah."
Him: "What, you agree that people are cunts?"
"Of course."
He immediately smiled, shook my hand, and told me I was alright. We ended up talking for about half an hour afterwards as he told me all the things I needed to do as a tourist.