Index: wp-includes/class-wp-embed.php
===================================================================
--- wp-includes/class-wp-embed.php	(revision 28397)
+++ wp-includes/class-wp-embed.php	(working copy)
@@ -308,11 +308,9 @@
 	 */
 	public function autoembed_callback( $match ) {
 		$oldval = $this->linkifunknown;
-		$this->linkifunknown = false;
 		$return = $this->shortcode( array(), $match[1] );
-		$this->linkifunknown = $oldval;
 
-		return "\n$return\n";
+		return strpos( $return, '<iframe' ) !== false ? "\n$return\n" : $return;
 	}
 
 	/**