There is a reason we fear working on legacy code. But, what is it that we fear when working on legacy code? It's not the code itself; the code cannot harm us. Instead, what we fear is hearing that we have introduced a bug. The most dreaded word that a developer can hear. A bug means that we have failed and that we will have to work on the legacy code again.

Exploring the types of issues we might run into while working on legacy code, we find several. Firstly, because we don't know the code, a change to one part might cause unintended side effects in a different part of the application. Another issue is the code could be over-optimized or written by someone who was trying to be clever when they wrote it. Lastly, tight coupling can make updating the code difficult.