Topic that must not be named
When Eliezer Yudkowsky established LW, he prescribed a moratorium on two topics until the end of April 2009. These are the singularity and Artificial General Intelligence. In discussions, these were often referred to as "The Topics that Must Not be Named". Occasionally you'll also see "The Institute that Must Not be Named." This is the Machine Intelligence Research Institute, known at the time as the Singularity Institute.