A case in point, a very good friend of mine races motorbikes and has done so in the Isle of Man a couple of times. After returning home he managed to find a photographer that was set up at one of his favourite places on the track and searched for his photograph so he could buy it. He soon discovered that he wasn't on the website so dropped the photographer a quick email. Turned out the photographer had a shot that was slightly out of focus and wasn't 100% happy with it, so didn't include it on the website. After receiving the image the good friend was over the moon. No, it wasn't a perfect image but it was him, doing what he loved in a place he loved and that meant more to him than any technical aspect of the photograph.
There's 4 ways this could go though:
1. They notice it's out of focus and are annoyed with you, but they have the image.
2. They don't notice and it's a shot of them they're happy with of them walking down the aisle as newlyweds.
3. You don't include it, and they wonder where the hell is the image of them walking down the aisle as newlyweds.
4. You don't include it, and they never even wonder why they don't have it because of all the other wonderful images you've delivered
Printed out in an album, it may be even less noticeable but if they decide they're having it on the wall you might be in trouble.
I would be tempted to include it personally, as Danny said as long as it conveys the moment, the relevant emotion and isn't massively offensive it should stay. 1 image (of an important part of the day) out of a couple of hundred supplied would be acceptable, supplying an entire wedding of missed focus shots wouldn't be. Making a habit of it would be dentrimental to your reputation, but I'm sure you already know that! In a perfect world it wouldn't have happened in the first place, but that's not really helpful at this point