This has been an enlightening conversation. Here's my story: I recently longarmed a quilt for a friend. The alternate blocks were 8" panel squares to which she had added a pieced border of half-square triangles, with the triangles pointing outward. When the quilting was done, I put it on my bed for pictures. I uploaded the pictures, and right in the middle of the first one was one of the panel squares with one pieced border of triangles pointing inward. It was glaring in the picture, but neither of us had noticed it during the piecing or the quilting.
When she came to pick up the finished quilt up, after she had looked at the quilting design and was quite happy with that, I told her I had something to show her. I apologized for not catching it while I was quilting, and she completely understood - after all, she'd been up close and personal to the quilt herself and hadn't seen it. I told her if she wanted to remove the quilting stitches and flip the border so it was like the others, I'd be glad to requilt that part of it for her.
After thinking about it, that's exactly what she decided to do. She plans to enter it into the local quilt show and didn't want the mistake to be left in the quilt. I'm glad I said something to her, and we joke about it, because that's what friends can do. If someone else had pointed it out to her, she might have been offended, embarrassed, or otherwise upset about it.
As for myself, I'd want to know about it, but I'd prefer the mistake be mentioned to me in a PM rather than publicly.