there was no way to 'manually' stop an animation

src/effects.js

@@ -470,7 +470,7 @@ jQuery.extend( jQuery.fx, {
 
 		if ( !timers.length ) {
 			jQuery.fx.stop();
-		} else if ( jQuery.support.requestAnimationFrame ) {
+		} else if ( jQuery.support.requestAnimationFrame && timerId) {
 			window[jQuery.support.requestAnimationFrame](jQuery.fx.tick);
 		}
 	},