Nine thousand baskets

The mistake

Every machine in the company, from office workstations to the shop's storefront servers, is configured to apply updates and reboot at the same wall-clock time, 2 PM, because that's assumed to be the quietest hour. Nobody staggers the schedule or checks it against actual live traffic, so the trigger fires everywhere simultaneously, including on the production system serving active shopping sessions. The result is a synchronized fleet-wide restart that drops every in-progress cart at once instead of a harmless maintenance window.