I was obviously joking about my nose moving the diopter slider, however I've hit all kinds of back buttons with it.
As far as how you move the slider over mistakenly, I'll have to "recreate" the moment tonight, but I am almost sure it happens when I'm running around the reception hall switching lenses. As I use a hand strap, I typically grip the camera with my left hand, let go of the hand strap, and switch out the lens with my right hand (lens belt pouch on the right).
I think my thumb naturally wraps around the flash hotshoe area for better support of the camera and as I twist/lock the lens, the slider moves. The eye piece does nothing to stop my big fingers from moving it.