Have you ever been reading or watching something, and just when things were starting to get interesting, you found yourself asking: “What?  Why didn’t they do ______?”  Sometimes, there’s a very good reason for this that will be discovered later, or the creator made a conscious decision for the character to make a mistake in that instance, or perhaps they were even limited by more practical considerations (in the case of movies or television) like special effects budgets and capabilities.  Regardless, these dichotomies, where you think something could have or should have happened, but it didn’t, can be terribly disruptive to a story.