Costco's website goes down for several predictable reasons: scheduled maintenance windows (often overnight or early morning), flash sales or promotional events that spike traffic beyond server capacity, payment gateway failures, CDN (content delivery network) disruptions, or DNS propagation issues after infrastructure changes. The most common cause is traffic overload. When Costco runs limited-time deals on high-demand items like electronics or bulk household goods, concurrent user sessions can overwhelm their load balancers. Retail sites handling membership authentication, inventory checks across warehouses, and payment processing face more failure points than simpler e-commerce setups. A single bottleneck in their API layer or database queries can cascade into a full outage. Scheduled maintenance is the second likely culprit. Enterprise retailers typically push updates during low-traffic windows—2 AM to 6 AM in their primary markets—but these can run long or encounter rollback issues. Costco operates across multiple time zones, so "overnight" maintenance might hit during business hours for some Canadian or international users. To verify if an outage is widespread or localized to you: check downdetector.com/status/costco, search Twitter for "Costco down", or try accessing the site from a different network (mobile data vs. home wifi). Browser cache or cookie corruption can mimic an outage. Clear your cache or test in incognito mode before assuming it's their problem. At Ottawa SEO, when we audit enterprise clients, we stress the importance of status pages, CDN redundancy, and horizontal scaling for traffic spikes. A site Costco's size should have geographic load distribution and auto-scaling, but even sophisticated setups fail under coordinated load or zero-day vulnerabilities. If you're building any membership or inventory-heavy platform, budget for 3–5× your expected peak traffic and run quarterly load tests. Costco's outages remind us that uptime is a function of architecture, not just good intentions.