Shipping & Returns - Warning: session_start(): Cannot send session cache limiter

[Lucky;)]

When I call page "Shipping & Returns" the Warning popups:

 

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/mysite/public_html/catalog/shipping.php:1) in /home/mysite/public_html/catalog/includes/functions/sessions.php on line 67

 

Why dooes it happen? When I open other pages I call "application_top.php" where"session" does init & start.- nothing wrong happens.