Ask HN: What are the common ways in which security fails in agile development?

As part of a conference I've been invited to present in, I've been asked to talk about integrating security into agile application development and I want to be able to talk about common ways in which security had failed in Agile development. Apart from the most obvious problems of not integrating security into DevOps or not testing the dependencies, do you guys know of any case studies of companies that have made large mistakes (security-wise) when building applications in an agile manner?