Elliot,
I switched to Twenty Thirteen and only have ACF activated. This is the error I get
[17-Sep-2013 13:12:29] PHP Fatal error: Call to undefined function filter_var() in /var/www/hlacharterschool.org/wp-install/wp-content/plugins/advanced-custom-fields/core/controllers/everything_fields.php on line 266
[17-Sep-2013 13:12:29] PHP Notice: ob_end_flush() [ref.outcontrol]: failed to delete buffer zlib output compression. in /var/www/hlacharterschool.org/wp-install/wp-includes/functions.php on line 2765
