Headers already being sent

[eluser]Unknown[/eluser]
I'm currently working on a project and using codeignter. In some of my controller functions I'm using redirect but when I hit these redirects I keep getting the following error message

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at C:\pathtoproject\system\core\Exceptions.php:170)

Filename: helpers/url_helper.php

Line Number: 551

I was just wondering what constitutes as headers already being sent and are there any tell tail signs I should look out for?

Thanks for any help

[eluser]weboap[/eluser]
look for space before <?php