taken from: -
Nikkor AF-S DX VR 18-200mm 3.5-5.6G IF-ED DSLR lens full review Cameralabs features
"The Nikkor lens didn't have a lock, but again could also be pointed directly up when zoomed-in to 200mm without falling back. If you were to zoom to a mid-way focal length on the Nikkor though, such as around the 100mm mark, the barrel can retract or extend under its own weight if pointed directly up or down; we have a demonstration of this in our Nikkor 18-200mm video tour. It can be annoying, but again only really affects the lens when pointed directly up or down from a mid-way focal length, and to be fair, it's par for the course for a lens with heavier internal components."