Because I had double-click-home set to the camera, this meant that every other time I started my iPhone, it was in the camera app, which was really confusing a lot of the time. Once I started installing the iOS4 betas, though, and double-click-home became 'reveal task switcher', her double-triple-quintuple clicking the home button did nothing but show and hide a row of icons.
After the first time she did that and I made fun of her, she learned pretty quick. Still, this sort of thing happens to the smartest/most capable/most educated people. It's not just mom and dad.