You can't use epub:type on an inactive link like this:

<a epub:type="glossref">foo</a>

If you do, epubcheck will report:

Error while parsing file: element "a" missing required attribute "href"

I don't know if this occurs often, but it seems like it might be too strict. I'm more worried that people have used it on bad markup patterns like this from the samples: <a epub:type="keyword">bar</a>.