I am reading a book about System Testing. It says the bugs will be duplicated across to the system and different project. I think this is because the code is re-using everywhere and sometime, we do some cut and paste. The worst case is the bug in the cut-paste code. As I remember, my thesis used a lot of cut-paste code too. I know this is not a good practice. That is better now. I am a C# programmer and learned to use component technologies. If there is in a component, it just needs to replace the DLL across projects. Yes, I know that is still a painful job.