Taking censorship to a new level, China has blocked Foursquare on the Tiananmen Square massacre's 21st anniversary because too many users checked into Tiananmen Square.
The Tiananmen Square massacre happened on June 4th, 1989 where various civilians were protesting Chinese communism and were killed by the Chinese government. The issue is still a sore subject for the government today. All information about the event is limited to the government, even Google has agrees to censor any information about the event. Last year, in the days leading up to the anniversary, Twitter, Facebook and Hotmail were blocked.