There are things you do not talk about at the dinner table. You know, politics, money, religion, and sex. This rule well applies at parties, weddings, etc. What is the touchiest and/or most depressing topic you can talk about at a party?
For me, they are:
World Trade Center/September 11, 2001
Natural Disasters, especially really bad ones like Hurricane Katrina or Tropical Cyclone Nargis
Joseph Stalin/Adolf Hitler