Help : Cannot modify header information - headers already sent by

[eluser]Asinox[/eluser]
hi, i dont know why this happen but.... i hav this error:

somebody know why?

Thanks u

Code:

A PHP Error was encountered
Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /home2/pasoapas/public_html/system/application/controllers/main.php:1)

Filename: libraries/Session.php

Line Number: 315

[eluser]Asinox[/eluser]
im sorry...just was a empty space before <?php ...

Thanks all