In my life I've seen that failure can ruin chances of success and for some and for others it can be a learning moment. Failure or success is in the hands of the individual and we also must remember while often there may be some outside factors affecting a person I do believe that in many cases one's failure is a result of their actions, in-actions and unfortunate certain outside issues that are not in their control.
You have people who know that drugs are horrible before they even begin them, but still they play with fire and ruin their lives (and the lives in their families and communities) while saying they'll quit...tomorrow, the next day, next week, month, etc. There are people who know other crimes are against the law but commit them anyway and get caught, repeatedly. You may know of a mooch that has accomplished nothing in life, living at home leeching off the family but contributing nothing. They are a failure and those who enable them are enabling a failure to their very own detriment as well.
On top of the individual's choices there are outside factors that are not within their control as well. This is why it is so important to do your best in each specific situation and inform yourself, ask questions, prepare, etc. When this is not possible you are limited to past experience, instinct, etc. which may or may not work in your favor. Not all things will work in your favor, well prepared or not however, for things most important to you, it's important to prepare to win. Not preparing/not planning is asking to fail.