All the women at my work have to wear skirts (or dresses, of course) and hosiery. Often, they'll wear knee-highs, the tops of which show when the walk--and worse still, sometimes all the time. It's like they don't even look in the mirror before they leave the house! If there's a possibility that the slit/walking vent in your skirt will show your knee-highs, just don't wear them. Frankly, I always feel a bit frumpy in knee-highs, anyway. I think I own two or three pair, all leftovers from my mission (which ended three years ago, which shows how often I wear them!).
Along the lines of exposed underpinnings, I can't tell you how many times I see slipped slips. Again, if the slit is higher then the hem of the slip, don't wear them together! Most skirts don't need slips, anyway. It's largely older ladies who do it, because slips are a generational holdover. My coworker is 25 years older than I am, and she apparently is scandalized when she forgets to put on a slip.
The worst, though--the absolute worst--are those rare souls who manage to expose both the bottom of their slip and the top of their knee highs at the same time. At that point, you have utterly failed to dress yourself. Most of these women are married; can they not ask their husband, "Honey, is my slip showing?" or "Do my knee-highs show when I walk?" I mean, is it so incredibly difficult to make sure one's underpinnings stay underneath one's clothes?