Wal-Mart experienced an online retailer's nightmare-come-true on Black Friday, as its website bit the dust for a large portion of one of the busiest shopping days of the year.

Early in the morning, visitors to Wal-Mart's website saw blank pages or delays, and attempts to place orders generally resulted in error messages. By later in the day visitors were told simply to "come back later," writes the Associated Press (via MediaBuyerPlanner). Walmart.com spokeswoman Amy Colella blamed traffic surges that were higher than anticipated.

Walmart.com is the nation's 21st most popular website, according to comScore Media Metrix. Keynote Systems Inc., a company which monitors performance at leading websites, said it began noticing problems at about 4:30 a.m. EST. Visitors could still access the site, but tasks that generally take a second or two were taking more than half a minute. Attempts to pay often resulted in error messages.

Amazon.com's website experienced a disruption on Thanksgiving Day, for about 15 minutes, beginning around 2 p.m. The disruption came as a result of a sale the retailer was offering: the Xbox 360 for $100 ($200 below the regular retail price) to the first 1,000 customers. The Xbox sold out in 29 seconds, according to an Amazon spokesperson.