If you use product reviews I may have stumbled on a minor 'bug':

It is entirely possible I'm incorrect, but I think part of the file includes\classes\observers\nmx_structured_data\class.breadcrumbs.php

doesn't validate (http://www.w3.org) because this code: on line 48:
'<meta itemprop="author" content='.$reviews->fields['customers_name'].'">'.

should be:
'<meta itemprop="author" content="'.$reviews->fields['customers_name'].'">'. ​​​​

The change is an addition of a " at the end of content=