Certain OPF errors not catched by Validate
If the </metadata> is accidently removed in the OPF, the Validate option does not report an issue. However, the OPF is no longer valid of course.
It causes that the ePUB cannot be opened in ADE and others.
I think it is a bug and I reported it on the site.
Fortunatly epubcheck does catch the error, so there is a work around.