Fix is_preview() to work properly when updating in admin AND when previewing in front end.

wp-preview-changes-meta-fix.php

<?php

// post meta for previews
class wp_preview_meta {

	private $doing_preview = false;

	public function __construct() {
		add_filter( 'add_post_metadata', 	array( $this, 'add' 	), 10, 5 );
		add_filter( 'update_post_metadata', 	array( $this, 'update' 	), 10, 5 );
		add_filter( 'delete_post_metadata', 	array( $this, 'delete' 	), 10, 5 );
		add_filter( 'get_post_metadata', 	array( $this, 'get' 	), 10, 4 );
	}

	public function is_preview() {
		if(is_admin()) {
			return !$this->doing_preview && isset($_POST[ 'wp-preview' ]) && $_POST['wp-preview'] == 'dopreview';
		}

		//And on the front end:
		return !$this->doing_preview && isset($_GET['preview']) && $_GET['preview'] == 'true';
	}

	private function mod_key( $key ) {
		if ( strlen( $key ) > 50 )
			$key = md5( $key );
		return "_preview__{$key}";
	}

	public function __call( $method, $args ) {
		if ( ! $this->is_preview() || ! function_exists( "{$method}_metadata" ) )
			return $args[0];

		// replace $check with $meta_type
		$args[ 0 ] = 'post';

		// modify key
		$args[ 2 ] = $this->mod_key( $args[ 2 ] );

		// call original function but make sure we don't get stuck in a loop
		$this->doing_preview = true;
		$result = call_user_func_array( "{$method}_metadata", $args );
		$this->doing_preview = false;

		return $result;
	}

}

$wp_preview_meta = new wp_preview_meta();