I have a client website with a custom-developed plugin that when data is updated (this is all in Wordpress admin backend) uses a PHP header('Location: ') function to switch to a different URL. I had originally developed in an environment with an old theme that I eventually scrapped for a newer better one, but now PHP throws an error saying:
PHP Warning: Cannot modify header information - headers already sent by (output started at /wp-admin/includes/template.php:2018) in /wp-content/plugins/*my-plugin-name*/save-data.php on line 47
which is where my header() function is in my plugin.
I originally was convinced it was a plugin conflict, but then I realized it was doing with the twenty fifteen theme and then any other theme I tried other than the crappy old one that was the active theme when I developed the plugin.