In Rails 5, the old way of returning false to implicitly halt a callback chain will not work anymore. This change will impact any codebase using ActiveSupport, ActiveRecord, ActiveModel or ActiveJob. Methods like before_action, before_save, before_validation will require developers to explicitly throw an exception in order the halt the chain. This talk will explain the motivations behind the new default, will delve into the internals of Rails to show the actual code, and will help developers and gem maintainers safely upgrade their apps to Rails 5.