During my last game (Emperor, tech victory, playing the Greeks) I managed to build the Manhattan Project, giving me a free nuke. Some turns later, I got the report that the Egyptians were 7 leaders/wonders away from winning a cultural victory.
Since I was in a democracy, congress prevent me from using the nuke. I went into anarchy, launched the nuke on Egypt's main city (5 wonders + 2 leaders, it wasn't their capital) completely wiping it of the map. The result? Egypt was set back severely in their attempt to win a cultural victory, and I suffered NO consequences. Nothing at all. I wasn't at war with the Egyptians. Nuking their city apparantly doesn't trigger a war. And since I was in Anarchy for one turn, I suffered no cultural penalty either. Next turn I switched back to Democracy, happily sold my Atomic Theory tech to the Egyptians I had just nuked, and went on my merry way to win the Space Race.
This has got to be an unintended bug. Other than having to be in anarchy for a mere 1 turn, there are no drawbacks for using a nuke.