Most web applications have a lot of before/after hooks that occur when working with objects: sending a welcome email on registration, incrementing/decrementing counter caches, trigger validation on remote web services. When implemented using callbacks, all these occur without the developer knowing about them. A simple change in one area of the code can have a huge impact somewhere else. Inspiration for this came from http://blog.teksol.info/2010/09/28/unintented-consequences-the-pitfalls-of-activerecord-callbacks.html and http://jamesgolick.com/2010/3/14/crazy-heretical-and-awesome-the-way-i-write-rails-apps.html