I've perpetrated one which I felt crummy about. To set the background, I was a sophomore at college in 1986, and the campus is close to some bad neighborhoods. I had had a very scary incident involving several black people while I was riding through one of those areas. So later I'm standing inside the college gate when three teenage black guys come up and start asking me to let them in. Unfortunately, they had a strong accent (from Florida as it turned out) and I couldn't understand most of what they were saying. So I wasn't letting them in...and they were getting pretty pissed. At any rate pretty soon an older student came by, who understood immediately what they were saying. It turned they were relatives of someone I knew, but the way they pronounced her name I just couldn't get it. No doubt it's also because I was nervous and primed with racist preconceptions (which, as I noted, also had some basis in prior experience). Anyway he let them right in and I felt pretty darn bad about it. You come all the way from Florida to visit your cousin at an Ivy League school, and some white kid thinks you're there to steal stuff. (One irony is that later that same year I did witness a black person steal our neighbors' TV...I was so surprised, just watched him carry it down the stairs and out the gate.) All in all I would say that racism weaves a tangled web of preconceptions that can be reinforced by actual events that also are ultimately arising from racist history.