Poor harry is in such trouble over the use of the word "negro."
The word negro is an anachronism, not a slur. There is another slur that begins with the letter "n" that has always been recognized as a slur both by the people that used it and the people that it was used against (Rappers are somehow exempt from all this). The word negro has never to my knowledge been a slur, it is simply a word we don't use anymore.
There are no hard and fast rules. That is what makes these standards of "sensitivity" so useful. Every time someone transgresses one of the many unstated rules they get to pass judgment based on a "wholistic" assessment. That is, they have discretion over when the penalties will be extracted.