Debugging
You have introduced a sneaky bug of failure in you program, the next step is to try to find out what that problem is so that you can fix it. This process is called debugging.
Debugging is the process of hunting down failures or bugs in your program and fix them. sometimes bugs can be obvious, and other times they can be profoundly tough to figure out.
When you build a system that has no test and you create breaking changes in in your code, it will be quite tough to hunt these bugs down, thus the need for unit and integration test.
When you face bugs, it’s better if your program does not interact with the outside world. you can have a crash from time to time, and you would surely bare with these changes. In other cases, if you have a program that communicate with the outside world, you might propagate the errors elsewhere and you surely don’t want to have a system to crash or provide false input to other systems.