Angst? What Angst?: A complaint that was raised when Mai, Natsuki and Nao seemingly choose to allow the greatest tragedies in their lives to go forward in order to avert a temporal paradox in a trip to the past. The following chapters, however, indicate that the experience has forced them to realize how much those events have defined them and reconsider their priorities.
Moral Event HorizonDiscussed in the third chapter, titled this, which states that actions that should qualify as this for Mary Sues often do not in the eyes of their authors, and are not portrayed as such. Hitomi proceeds to cross it by using mind control to force a police officer to murder 20 innocent people, then commit suicidewhen he's done.
Shizune crosses it in her first appearance, by murdering Toki.