A big part of this advice boils down to the old adage: "Don't remove a fence if you don't know why it was put there." In other words, when making changes, make

A big part of this advice boils down to the old adage: "Don't remove a fence if you don't know why it was put there." In other words, when making changes, make sure you preserve every behavior of the old code, even things that seem unnecessary or counter-intuitive.