A lot of the time i am sure people (myself included) put their foot in it unintentionally, when its done on purpose its a totally different thing and can be very hurtful for the person that it is aimed at. Its basically a form of bullying and those that do it are the ones that shout loudest if it is done to them.
Some seem to be able to do it all the time and no one says a word, others say hardly nothing and are jumped on immediately and made to feel bad. We all say all the time if you dont like someone just dont speak to them if you really feel you have good reason.
As Kenty said few of us know the anyone for real!!!