Shares of Apple and Amazon tumbled in extended trading after both reported disappointing quarterly results. Amazon is suffering because of a surge in the cost of labor and fulfillment, while Apple is taking a hit because it can’t meet demand for its products. It’s a sign that even the mightiest companies are stumbling over the world’s strained supply chains, and that it’s going to be very difficult to meet demand this holiday season. Amazon plans to spend billions hiring more workers, raising wages and speeding deliveries at a cost that could eat up its fourth-quarter profits.