Response:
You may know where it’s going wrong and what data is causing it to go wrong but not why it’s being given the invalid data. You may need to look further afield in your investigations.
This question could easily be a follow on to “do you know what’s wrong?”.
One way to debug a program is to use a debugger. Another, is to trace its execution.
See Wikipedia: root cause analysis and Wikipedia: debugging techniques.
Reference: