A story or series of stories have established a core group of heroes, villains, and a supporting cast. In order to add more depth to one or more of these things, the creator(s) put(s) out fully-developed back stories and prequels, or even interquels. While such efforts may produce many great things, they also doom characters and plots to inevitable failure or success. Why? Because the first story or stories established de facto canon. This trope is not about someone who met his or her demise by cannons (regular or otherwise), you want There Is No Kill Like Overkill for that.