To laugh at someone's suffering is indeed wrong - always. That being said, there are times when people laugh not because they think something is funny, but because it is a nervous response or reaction. Those instances are somewhat understandable, but to find humor in another persons pain I think is indeed unacceptable.
I think yes and no. There are circumstances. That it won't cause any harm. Things that are small enough not to actually bother anyone. We laugh at our own misfortunes. Yet, there is a fine line between the two. If your friends trips and falls on their feet. That's OK. If they loose their home, that's not an OK thing.