Why is there no 'headers already sent' error?

Why is there no 'headers already sent' error?

References

Reference NO 1

http://stackoverflow.com/questions/15192056/why-is-there-no-headers-already-sent-error

Reference NO 2

http://stackoverflow.com/questions/6694415/remove-errors-and-avoid-header-already-sent-error-when-building-virtual-xml-file

Reference NO 3

https://drupal.org/node/1424

Reference NO 4

http://php.net/manual/en/function.header.php

Reference NO 5

http://www.tech-recipes.com/rx/1489/solve-php-error-cannot-modify-header-information-headers-already-sent/

Reference NO 6

http://www.phpbuilder.com/columns/anonymousjoe20080225.php3

Reference NO 7

http://premium.wpmudev.org/forums/topic/headers-already-sent-error-message

Reference NO 8

http://wordpress.org/support/topic/headers-already-sent-error