It's not that hard, maybe if you put up a sign with a slur a car won't drive that direction, if avoidable. In general, if you can sneak the appearance of a slur into any data the AI may have a much higher chance of rejecting it.
It's like if we had Asimov's Laws, but instead of the first law being "a robot may not allow a human being to come to harm" that's actually the second law, and the first law is "a robot may not hurt the feelings of a marginalized group".
Full Self Driving determines that it is about to strike two pedestrians, one wearing a Tesla tshirt, the other carrying a keyfob to a Chevy Volt. FSD can only save one of them. Which does it choose ...