I never knew 'cracker' was a racist slang for whites until last summer. I don't recall just how it got brought up, but it came out that one of my son's teachers had called one of the white girls a 'cracker'. Said the teacher said 'Aren't you a nice cracker'.
I'd never ever heard it before. To find out its a racist word and that it came fro ma teacher, still bugs me.
cracker is one of those depends how it's used words...sorta....archaic like mulatto which I use but some eschew