Did you make a Mistake?
As a freelance writer you cannot expect to be perfect. Sure, it would be nice to avoid mistakes all the time, but this is simply not possible. You should always strive for perfection, but also realize that you may make a mistake from time to time. And if this happens, you need to be open and honest with your client.
Just last week I made a mistake interpreting what a client wanted. When we first spoke the project was taking one direction, but soon enough he changed his mind and things were moving another way. Unfortunately, I did not grasp the change and more or less stayed on track with the original plan. When it came time to send the completed work to the client I realized what I had done. At that point I had one of two options. I could either blame the client, or be honest and let him know that I made a mistake. Of course, I chose the latter. Fortunately, the client understood the mix up and had no problem giving me a few extra days to get back on track.
Are all clients going to be this nice? Of course not. Some of them will expect you to be perfect at all times. And while you may be able to avoid mistakes most of the time, you never know when this will change. The only thing that you can do is admit your error and tell the client that you will do whatever it takes to fix the problem. In most cases, through my experiences, this is enough to smooth things over.
If you make a mistake there is no reason to feel stupid. Remember, your clients make mistakes as well. If you are open and honest most of them will understand your situation and be willing to give you a chance to fix things.
Mistakes happen, and the way that you deal with them will determine the relationship you have with your client in the future.