Plugin debugging - tracking down "The plugin generated xxx characters of unexpected output during activation." errors. I didn't invent it, but found it on a few places on the web, including wp.org support forums.

sgr_wp_debug_activation.php

<?php
add_action('activated_plugin','save_error');
/**
 * Place entire block of code at foot of wp-admin/includes/plugin.php
 * Don't forget to remove code after use!
 *
 * Location of error_activation.html must be writeable, eg in your uploads folder somewhere
 */
function save_error(){
  file_put_contents( ABSPATH . 'wp-content/{somewhere writeable}/error_activation.html', ob_get_contents() );
}