This was not the day to be a Walmart shareholder. Stock of the venerable retailer tumbled 9% after the company reported a dip in comparable sales for its U.S. stores in its fiscal second quarter ending July, even as the e-commerce part of its business grew 23%. The results were fine, really, ...

Walmart shares fell 8% despite strong second-quarter results, with revenue rising 5.9%, e-commerce sales jumping 23% and operating income increasing 28.8%. Investors focused on…