Especially when the leo is low-back.  There's less fabric there to hold on to the shoulders.  But I love the low-back look better than anything.  I wear adult size large, and when I first put on a lycra low-back leotard, it tends to slip off the shoulder easily.  But once I start exercising, that takes care of the problem.  Just a slight amount of sweat moisture on the skin is enough to make the leo cling faithfully to my body.