There are SO many words one shouldn't say to another, and here, I've listed down some words and to whom you shouldn't say them to! ;)

Word/s: B*tch
Never use on: girl friends, GIRLFRIENDS, macho men.

Word/s: I don't give a (insert foul word here) about you!
Never use on: parents, best friend, other dear relatives.

Word/s: "You're just a pebble in my shoe." (From Ever After)
Never use on: people close to you, grandparents

Word/s: F*ck Off
Never use on: ANYONE! That'll hurt more than being back-stabbed.

Frankly, these are the only words I'm aware of. And frankly, I've never, and will never, use these words on anyone. Human emotions are too fragile to be trampled on this way, and wounds wouldn't have to heal if they  weren't there in the first place.