Sometime, when you try to activate a plugin, you may find an activation warning/error like this

The plugin generated xxxx 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.

Since you can only see that message, you will get clueless since you don't know what is the unexpected output. Plugin developer will hard to trace what is going on since they don't get enough information.

To display the unexpected output, you can use the modified version of WordPress plugin manager attached to this topic/post (find it below). You just need to extract this plugins.zip and upload plugins.php to wp-admin (overwrite the existing file).

Note: login to this forum is required to download the attachment