Well I can tell you not to fret. I have seen professional projects with this same problems. When fixing X I break Y and Z. This is generally caused by one of two things. Lack of RCA (root cause analysis), the programmer is not looking deep enough to find the true issue he is finding the first indiction of an issue and patching it up. The second is aggressive coding, the programmer is attempting to do too much with a class or a method. This creates a lot of dependencies and when it comes time to make changes disaster is the result.