gistfile1.php

<?php
//suppose this is the _doing_it_wrong custom column
add_filter('manage_users_columns', 'pippin_add_user_id_column');
function pippin_add_user_id_column($columns) {
	$columns['pippen_id'] = 'Pippen ID';
	return $columns;
}
 
add_filter('manage_users_custom_column',  'pippin_show_user_id_column_content', 10, 3);
function pippin_show_user_id_column_content($value, $column_name, $user_id) {
	$user = get_userdata( $user_id );
	if ( 'pippen_id' == $column_name )
		return $user_id;
	// *should* return $value here
}

// this does it right, and works with for _doing_it_wrong above
add_filter('manage_users_columns', 'ipstenu_add_user_id_column');
function ipstenu_add_user_id_column($columns) {
	$columns['ipstenu_id'] = 'Ipstenu ID';
	return $columns;
}
 
add_filter('manage_users_custom_column',  'ipstenu_show_user_id_column_content', 11, 3);
function ipstenu_show_user_id_column_content($value, $column_name, $user_id) {
	$user = get_userdata( $user_id );
	if ( 'ipstenu_id' == $column_name )
		return $user_id;

	return $value;
}