"In stressful situations you will not rise to the occasion, but rather fall to the level of your training". When it is time to react to a defensive situation your mind and body will respond in the way it was trained to.
When it comes to firearms handling...anything that can go wrong, will go wrong. Are you prepared?
Many people purchase a firearm "just in case." They put it somewhere safe (hopefully), and assume they will know how to handle their firearm effectively, when the time comes. Unfortunately, it doesn't work that way.
Hopefully you will never find yourself in a situation where you are forced to use a firearm for protection. But if you do, you will revert back to your training or lack thereof. The chances of something going wrong with your firearm in a normal situation is high, nevermind a life-or-death situation.