I would suggest reading some books on Seeing Whiteness.
I have been amazed seeing whites say things like, "I thought you people had rhythm," or "Mexicans are taking our jobs" (at McDonald's? Cleaning our toilets?), etc.
But racism is often even more subtle.
Someone will say, "i was in an elevator with a woman and a black guy, and it was funny because we were all quiet, just staring up at the numbers."
What race was the woman? White. She was referred to as "a woman" but the race of the man, although not relevant or important, was differentianted. Why? Because he was seen as "other."
As a gay man, I can relate when someone calls and asks for Mrs. Beanieboy. I say, "My mom?" They say, "no, your wife." I say, "HE'S not here right now."
Were I to call a straight person, and ask if their boyfriend was at home, wouldn't they be offended? Then why should I not be when someone assumes that I am hetero?
I don't think people of color enjoy being thought of as "the black guy" or "the asian guy" or "the latin guy." My Asian friends have told me on a number of occassions of ignorant white people that say, "HELLO! CAN YOU SPEAK ENGLISH????" to which they say, "Yes, and I'm not deaf either!" Many of my black friends, ones who are accomplished, have Phd's, talk about DWB - being pulled over for Driving While Black.
Now, put yourself in that position, where people are always treating you like that, and you will understand why you, at times, might encounter their frustration - because they put up with it all the time.