Humor Limits

When is it o.k. to draw humor from a subject or situation that is painful to others? Are there certain things that simply should never be funny due to the subject matter? Or is it o.k. to laugh at anything as long as you are not exposing the people who are sensitive to the subject of the humor?

I think that is the case, but sometimes it seems certain subjects are just so taboo that they should never be made fun of. I am not sure about this, but I think everything should be fair game – as long as nothing gets back to hurt the person who was the victim of whatever negativity that took place.