When the band is too loose it rides up at the back and hangs too low in the front. I loved how this article called it the see-saw effect. I am going to use that term from now on. However when I clicked on the size calculator on that link, I couldn't figure it out at all. They didn't have the option of a 30 band, let alone 28. And I really dislike linking bra size to clothes size. In my first pregnancy I went up roughly 5 cup sizes, but went up just one clothes size -- I found that it was the cut, not the size of the top which mattered.
Interestingly, when I randomly look at Yahoo Answers bra problems, I see that a lot of women think a bra band riding up means it is too small. If you think about that it doesn't make any sense: if anything a too small band would ride down, because our waist are almost always smaller or at least more compressible) than our upper back. (Ok, in reality too small bands don't go anywhere, they just squeeze you like an anaconda, even when tried on backwards to make sure it's not the cups or underwires that are the culprits.)
The right sized (snug) band stays even and doesn't allow the straps to pull it up; and the fullest part of the breasts is midway between the elbow and shoulder.
When I see bedroom scenes in movies, I often spoil the atmosphere by yelling "BUT HER BAND IS TOTALLY RIDING UP!" Do any of you do that too, or is it just me? Also, am I right in noticing that TV people from UK seem to be better fitted?