Looks basic to me, something effective testing should have caught. Result, you lose 4-5 million customers, some forever.

Sears' Web Goof: 5 Million Customers Blocked

When Sears upgraded its E-Commerce site's infrastructure in late August, it inadvertently blocked all visitors who don't accept cookies, a move that the $44 billion chain is now attempting to fix.
Although both industry and Sears sources estimate that only 1.2 to 3 percent of U.S. shoppers block cookies these days, it could still prevent almost 5 million U.S. shoppers from giving any of their e-dollars to Sears. And it's another reminder of the unintended consequences of Web and mobile site infrastructure changes.