With all the changes that push an application towards the legacy realm, often the methods or functions in the application will be used in unexpected places, far away from the code that you are changing.

There are two primary violations of the SOLID principles that have led to this issue, and the same two can help you to avoid it going forward. The first is the OCP (Open Closed Principle), and the second is the LSP (Liskov Substitution Principle).