During activation of plugin the following "unexpected output" error is shown if the site is protected by HTTP Basic authentication (.htaccess)
The plugin generated xxx characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
With WordPress debugging enabled, the reason can be seen:
Warning: fopen(http://example.com/wordpress/wp-cron.php?doing_wp_cron=1350641669.2885849475860595703125) [function.fopen]: failed to open stream: HTTP request failed! HTTP/1.1 401 Authorization Required in /var/www/wordpress/wp-includes/class-http.php on line 923
The call to wp-cron should be "silent" in the plugin.