I recently updated some webservers to use PHP 5.4 from 5.3. For a few WordPress sites, this caused it to begin spitting out Warning messages on the website. The warning messages in some cases caused other issues because response headers were already written due to the error, etc.
While the real solution here is to refactor the code to not use deprecated functions, a simple quick and dirty workaround is to add the following directive to the wp-config.php file:
ini_set( 'display_errors', 0 );