Noticed that when using open graph meta tags (which Twitter cards should fallback on?), the parsing might fail if the property attribute is not the first attribute of the description meta tag. Example:
<meta name="description" property="og:description" content="Combine all your content, including what your fans say about you, into one social hub.">
results in:
ERROR: Invalid value (message: Field description failed to validate because: text expects tag property, but not found in Map().)
while
<meta property="og:description" name="description" content="Combine all your content, including what your fans say about you, into one social hub.">
will be parsed successfully.