'The word mong means Down’s Syndrome about as much as the word gay means happy. I never use the word mong to mean anything to do with Down's Syndrome.' Read more here
There's a fascinating article about Gerry Sadowitz, who has been known to offend even the most open-minded of his audience. But he makes the point that comedy needs to be thought-provoking. He says of Frankie Boyle: "He'll write a joke, or someone will write it and give it to him, and he'll do it without any thought."