As soon as I read that, I thought to myself "who in their right mind would enjoy offending and therefore upsetting someone?", but when you think about it, a lot of internet trolling is based on purposely offending people.
I actually don't think I ever have offended anyone. I might have done on the internet years ago, but I can't actually remember ever seriously offending anyone recently.
Edit: No wait actually, I've just remembered that when I went on holiday on a school trip (I think I was about 13/14), my friends and I were discussing how fat one of the girls in our year was (who was also on the trip) and her friend overheard us and went and told her. Then she proceeded to shout expletives at us. At the time I don't think we really cared.
Come to think of it, I was actually a pretty nasty 13/14 year old.