“Consider that you may never complete that refactoring, and think about what the application will look like with two different ways of doing the same thing. Will this aid those coming after you, or hinder them? What happens if your way turns out to be sub-optimal? Will replacing it be easy? Or would it have been better to leave the older, but more consistent code in place?”
http://mikehadlow.blogspot.com/2014/12/the-lava-layer-anti-pattern.html