Sometimes, when you activate a plugin in the admin panel, you may see an error message like this:

The plugin generated 1 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.

This is usually caused by additional white spaces or empty lines at the beginning and/or end in one of the plugin's PHP files. PHP files should start with the <?php tag in line 1 column 1 and end with a ?> on the very last line. Additional empty lines before the starting- or after the closing tag can cause this (basically harmless) error.

Correct

<?php
    [... code goes here ...]
?>

Incorrect (empty line after the closing tag):

<?php
    [... code goes here ...]
?>
     << empty line after the ending tag

Incorrect (emtpy line before the starting tag):

     << empty line before the starting tag
<?php
    [... code goes here ...]
?>

You should report this as a bug to the plugin author and can temporarily fix it by making sure that all the plugin's PHP files do have properly set starting and ending tags. The problem isn't serious, but it can cause Headers already sent errors and some other annoying things.