Speed Bumps – Example code to accompany http://fus.in/1MidK1N

first-functions-sample.php

<?php
register_speed_bump( 'rickroll', array(
	'string_to_inject' => function() { return '<iframe width="420" height="315" src="https://www.youtube.com/embed/dQw4w9WgXcQ" frameborder="0" allowfullscreen></iframe>'; },
	'minimum_content_length' => false,
	'from_start' => false,
	'from_end' => false,
));
add_filter( 'the_content', 'insert_speed_bumps', 1 );