Even with your best intention; even if you worded your words with careful thought; even if you said it without meaning any harm - the fact is, some people would certainly be offended.
There are of course people who seemingly talk nonsense, in what appears to be an attempt to offend others. They try to come up with some stupid or tasteless joke... which they might not realize doesn't sit well with others, and how some might find them offensive.
The point with those two situation is this: you certainly can't stop people from saying things which you might find offensive. Notice how I emphasis on the you part? I think it's pretty telling, judging from how we would say 'I am offended!' rather than 'your words are offensive!' (And even if it's the latter, to whom? To you, that's who!).
So am I saying that it's pointless in trying to fight to stop people from saying offensive things? Not really, just because being offended is a personal thing, it doesn't give license for you to shoot your mouth off. What we could do though, is choose the way to how we react to what others say. We have a choice in that - whereas we can't control the words which come out of others.
On a similar note, here's a post on how we get the concept of being offended wrong (Cracked article? No surprise there mate)