Everyone makes mistakes some time, but it is how you see these mistakes that will determine whether you will make the same mistakes later on. See each unexpected result as a chance to learn more about photography and its techniques. In fact, I would go so far as to say that if you are not making mistakes then you are probably not learning as much as you could about how to use your equipment and make better pictures.
The best approach when you receive unexpected results from processing or when viewing your day's shooting on your computer is to identify what went wrong. Was it the film, or the camera, or (most likely) the way ...