The first not serious rule of programming says: "don't touch if it works". It means that you should not spend much time checking things that proved to be reliable in a working system. But sometimes things can go wrong.
Every developer has to manage with bugs. We have to deal with this because no one is able to write code without errors. Requirements are always changing, systems becoming more complex with time and in such situations, it's hard to not make mistakes. But how to avoid repetition of the same bugs when you edit a code? There's a good solution that will also save you some time while debugging the reasons of bugs.