Regardless of whether or not tabindex is set, checkboxes are getting the attribute added.
line 45 of checkboxes.php sets
$atts['tabindex'] = $tag->get_option( 'tabindex', 'int', true );
which returns false from shortcodes.php line 252-253 regardless of the pattern that is passed to get_option if there is no matching attribute set
if ( ! $matches )
return false;however line 76 of checkbox.php is expecting a null string to be returned, not a boolean
if ( '' !== $atts['tabindex'] ) {
side note: re-using the bult-in function name get_option is rather confusing.