Wikipedia
You would think that of course Wikipedia is banned in China, it is a huge database of information that anyone around the world can access in many languages. While it was able to be accessed for some time, it was banned shortly after the anniversary of the 1989 Tiananmen Square massacre.
Wikipedia officials were confused and did not understand why the ban was happening now and they received no explanation from the Chinese government. But most people had a hunch why it happened because of the timing, plus the danger that Wikipedia could give the people of China information that could lead to protests.