I ran across a rather irritating problem this weekend.
Nook STR (and Color from what I've read), doesn't recognize:
<meta content="coverimg" name="cover"/>
...in content.opf. It insists on having:
<meta name="cover" content="coverimg"/>
...with the name attribute first.
Sigil and apparently caliber epub->epub conversion will 'correct' the problem. But since calibre is already modifying the epub to include the current calibre cover when it sends to the device, perhaps it could insure that order in the meta tag, too?