A PHP script with includes followed by commands which set headers (ie session_start) will generate errors if include file(s) contain blank lines. For example:

<?
  include "includes/functions.inc";
  session_start();
?>

where functions.inc contains leading or trailing blank lines will result in an error like:

Warning: Cannot send session cache limiter - headers already sent

This problem appears to be very common (according to a search on google), and has no doubt frustrated many PHP users.

Unfortunately this behaviour is technically 'correct', although it should probably be fixed anyway.