Adding a prefix or postfix to columns in the postscript region gives an undefined index error - e.g:

notice: Undefined index: postscript_four_classes in /var/www/sites/all/themes/omega/template.php on line 143.

which is this line:

$vars[$region .'_classes'] .= ' '.$attribute .'-'. $value;

My fix is

$vars[$region .'_classes'] = (isset($vars[$region .'_classes']) ? $vars[$region .'_classes'].' ' : '') . $attribute .'-'. $value;