Shares of Amazon.com Inc. AMZN, -12.24% tumbled 12.5% in morning trading Friday, enough to pace the S&P 500's SPX, -1.14% losers, after the ecommerce giant reported a surprise first-quarter loss and revenue that came up a bit shy. The stock, which was trading at the lowest levels seen during regular-session hours since June 2020, was on track for the biggest one-day percentage selloff since it plunged 12.7% on Oct. 26, 2011.