When I was a young girl, I had an autograph book. I have no idea whose signatures were in it or what happened to it, but I do remember what my mother had inscribed on the inside cover: "Throughout life, always remember what Thumper's daddy said – if you can't say something nice, don't say nothin' at all." Those words have always been one of the guiding principles of my life. Sometimes it is difficult – for example, if someone says something particularly vile or stupid, it's awfully tempting to respond in kind, or point out the obvious flaws in their thinking. Generally this turns out not to be such a good idea, because one thing leads to another and the next thing you know, you've gotten into a full blown argument with them (usually over something irrelevant or unimportant in the larger scheme of things) and you're both saying all kinds of things that aren't very nice (and no one's backing down). Better to have kept your mouth shut in the first place (or simply said 'You are entitled to your opinion, and I to mine' before walking away or changing the subject). .
Of course, there are also times when you're tired, feeling out of sorts, or a bit cranky, and you open your mouth and say the first thing that comes to mind – which is usually a reflection of your bad mood – and you end up hurting someone's feelings, or getting back what you give (cruel and angry words usually beget cruel and angry words right back at ya). Everyone has these momentary slip ups – when you say something you really shouldn't (whether or not you 'mean' it or 'believe' it is not the point – if what you say is hurtful, not helpful, honesty isn't a factor). Those occasional lapses – while not entirely forgivable – are understandable, as long as we don't make being malicious a habit, and we have the good grace to apologize for our verbal gaffes afterwards.