You have to be careful that what follows the "but" is not a personal criticisim. For example: "I don't mean to be rude, but you're an idiot" is no less rud

You have to be careful that what follows the "but" is not a personal criticisim. For example: "I don't mean to be rude, but you're an idiot" is no less rude as a result of claiming not to be rude (though they /may/ not notice this and it might work).