This statement in the item element's definition contradicts the requirements in the vocabulary:

EPUB Creators MUST declare all applicable descriptive metadata properties for each Publication Resource in this attribute (e.g., if the resource is the cover image, contains scripting, or references remote resources).

It's true of all but the cover-image, which doesn't have to be identified. This is the problem that typically arises when we make the same requirements in different places. It feels like we should make this informative (possible a note?) and refer to the vocabulary for the requirements.

Same goes for the next sentence that requires the nav property be set, which technically means we require it be set three times (the paragraph above requires it as does its definition).