My wife has a copy of the 18-200VRII and there's no question about the lens creep being, well, creepy. ; )
She keeps the camera strap over her shoulder, and if she's not careful the lens will work it's way forward on our hard hikes. As long as she has it locked, though, it never seems to be a problem.
Sorry to hear about your wasted shots in Istanbul, however if it will make you feel any better I ALWAYS wish I'd done things differently, had different equipment with me, approached this subject in a different frame of mind, or different time of day... never seem to get THE shot.
I think, too, that when I do manage to have all those lined up I'll be ready for my dirt nap. Hope it doesn't happen too soon!!!!
Besides, back focus issues with your camera or a faulty lens are precisely the excuse needed to revisit Istanbul. : )