No matter how many debug printfs you add to a program, they won't help you if you're editing a different copy than you are running. You will make this mistake a

No matter how many debug printfs you add to a program, they won't help you if you're editing a different copy than you are running. You will make this mistake and you will learn from it, but even reading this warning will not teach you, you must make the mistake yourself. If you are lucky, you'll lose only an hour. If not you'll lose a week.