A friend of mine has a physical handicap that severely limits his motor functions. It's actually pretty amazing what he has been able to do, as he uses the mouse with his left hand (still with great difficulty), while his right hand can do little more than make inputs with one finger. He has to work so much harder to do what we all take for granted and it is truly inspirational to see him in action.
I've seen him perform in some games to the point where you would never know that he was playing with such a severe impairment, but sometimes no matter how hard he tries, he makes a mistake. Every time I'm in a duty finder and someone messes up, I always think of my friend and imagine that many people out there are dealing with hurdles in life that we may not even be able to comprehend.
So next time someone does something silly, instead of calling them a retard, remember that they might actually be mentally handicapped and just trying the hardest they can to get some enjoyment out of life. Play with compassion and remember that there are a lot of people out there that are not as fortunate as others, but still try as hard and deserve respect not ridicule.