So sorry to hear that, but haven't we all done something similar at one time or another. If it took you so long to see it, your friends may not notice it at all. Many times board members post pics of their mistakes which most of us could not find until the error was explained. So maybe it is best not to bring it to their attention...especially if they are not quilters.
I like the tip about taking photos in hopes of seeing a mistake early enough to correct it. Also, I've heard that if you look at your quilt on the design wall with a mirror errors are easier to spot. That is, look like it as in a rear-view mirror.