I ran into this recently, one of my quilting buddies who is also a longarm quilter was showing a quilt that she was going to feature in an artisan show that weekend - I noticed that she had missed quilting a small area - she was thankful and said that she should let me look over the rest of the quilt to see if I could find anything else - she wasn't mad - she was grateful.
I think it is all in the telling ... I am always thankful when someone points out something like that to me - especially if it is early enough in the game for me to fix ... sometimes I am too close to the project (when it's mine) to see a block turned the wrong way, etc.
I make my quilts to be used and abused, not hung up and judged so if there is an imperfection I don't usually get too upset about them ...