This code was not working:childViewController.currentIndex = currentPageIndex*kGalleryPageSize + photoIndex;

When currentPageIndex was zero and photoIndex was 1, I got thirteen. Very strange. I took a look at kGalleryPageSize

kGalleryPageSize [self isKindOfClass:[UserViewController class]] ? 9 : 12

Originally, this was always 12, but becomes 9 sometimes in a newly added feature. The original code became, then, the following:

childViewController.currentIndex = currentPageIndex*[self isKindOfClass:[UserViewController class]] ? 9 : 12 + photoIndex;

It looks like currentIndex (zero), multiplied by the result of isKindOfClass: (which doesn’t matter, the result is zero), is then evaluated as a boolean (false) which yields 12, plus 1, resulting in thirteen.

Oops.