Invalid command 'Header' .htaccess file

If you ever see this error in your logs, and you are getting "500 internal server error" from the server when you try to load pages.

The problem is very simple to solve. If you are on Ubuntu do this:

sudo cd /etc/apache2/mods-enabled/
sudo ln -s ../mods-available/headers.load headers.load
sudo service apache2 restart

That is it. You have just solved it.