I don't think it is a common occurrence with the particular pejorative term we are discussing.
If I am correctly guessing the term to which you are referring then I have no frame of reference. I just know that it is inappropriate for me to use it because it would be considered harmful and insulting so I eschew its use.
People with mental disabilities come in all flavors and it is a disability that crosses all lines: ethnic, religious, political, cultural, economic, etc. It can occur at birth or later. I just think we can be big enough not to use their condition as an insult or as a joke.
I agree. I also happen to think it's harmful and insulting no matter who uses it. As it relates to this topic, as others have said, they would not use the "R" word when referring to someone that is disabled, but don't see it as harmful when describing the behavior of someone that is not disabled. That seems to be the same principle - ie. "intent" and context is what makes a word inappropriate.