Most important, thanks for the great plugin and the all the work you put into all the posts about seo!
Using the latest version of the plugin my site was giving 500 errors. I tracked it down to line 207 in wpseo_functions.php.
The 'get_the_terms' call returns an array of taxonomy terms indexed by term_id, not 0 based. When the 0 based reference to the term_id is passed into the field function the WP_error happens and caused the 500 error. Changing the function from get_the_terms to wp_get_object_terms seems to fix the problem. If you want to see what is generated i'll be pushing the fix to http://www.sparkle-gear.com for testing.