According to Ken Rockwell a feature of a lens which is supposed to be there.
According to everyone else it's dodgy design. It's when you grab a lens like the 18-200mm Nikkor zoom it to around 35mm and point it down only to see the zoom slowly extend under it's crap gearing and overly heavy front element. Same happens at around 180mm if you point it up it'll zoom in.
This is always a royal pain in the buttocks if you're trying to photograph something up or down.
Somes lenses, like Sigmas 17-70, have a lock button so you can keep the lens at a certain focal length, which is kind of an admission that it happens, instead of doing anything about it, but then again, it's not exactly the most expensive lens you can buy ;-)