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A bit like blackface. People don't know why it's wrong so they believe it's fine when in fact it's wrong. A bit like half-caste. "caste" is a class system, not a racial system. The connotations of half-caste is that the person is over a lower class. But not knowing that doesn't make a person deliberately racist or mean, but I would hope that once pointed out that it's taken onboard and not misused.
I once called an Englishman of Indian descent and I called him a Paki. I was shocked and the anger that generated. Another guy took my aside and explained what the term "Paki" typicaly meant in England. Despite knowing I had no intention at all to be racist, I went up, apologised and I have, done it since. I was ignorant.