adsanity_post_class.php

<?php
/**
 * Allows you to modify the css classes that are applied to an ad unit so you can style each unit individually
 */
function example_adsanity_post_class( $classes = array(), $post = false ) {

	// Adds a dynamic class that utilizes data from the post object
	if ( FALSE !== $post && $post instanceof WP_Post ) {
		$classes[] = sprintf( 'custom-%d', $post->ID );
	}
	
	// Adds a static class to all ads
	$classes[] = 'special-ad';
	
	return $classes;
	
}

add_filter( 'adsanity_post_class', 'example_adsanity_post_class', 10, 2 );

You have a typo on line 8.

Should be:

if ( FALSE !== $post && $post instanceof WP_Post ) {

Great catch, @djmtype! I've fixed the gist.