Ok, you have just finished squeezing the best of the best out of your brain. Finally you have been able to write down your ideas in programming language. The program is finished!
Now you are all excited and ready to show your new baby to the world, right? Wrong! Before you start screaming and inviting over everybody, test your product yourself.
Do not make the mistake to think that because you have put in all your best efforts and best thinking your product will work flawlessly. Try it out. Put yourself in the shoes of the end user. Make sure that you try out everything very thoroughly.
Do not assume that because you consider yourself some kind of programming genius that everything you spit out will work as expected. Do not trust yourself.
Make sure to deliver quality! No matter how small a change you make to a program, test it.