If it's not the bottom button, is there a tiny bit of bubbling at the curve of the bib? As the others have said, I had to really scrutinize. There is nothing glaring and it could be just the way it was hanging on the hangar.
You know, when I look at how things I used to buy fit me/looked on me, I wonder why I don't expect the same quality from RTW that I do from my own garments. Really, it's amazing how we can obsess if we make it!