Sears, Saks Fifth Avenue, Costco, Dell, Victoria's Secret, Bloomingdale's and Williams-Sonoma all saw their sites malfunction at some point between Friday and Monday, according to companies that monitor Web performance.

The day after the Thanksgiving holiday in the U.S., known as Black Friday, is a big shopping day and kicks off a major weekend for retailers that ties in with giant online sales on what is known as Cyber Monday.

Online stores that slow to a crawl, return error messages or collapse altogether are the bane of retailers, since those problems are a major turnoff for prospective buyers and, depending on their severity, can seriously hurt sales.

Most of the problems affected checkout processes, very possibly leading to lost sales at a point when customers were about to make a purchase, said Shawn White, director of external operations at Web monitoring company Keynote Systems. "Checkout is obviously the most critical part of the online shopping experience," White said.

This problem reflects a common oversight on the part of online retailers: load testing their entire e-store, not just the home page, he said.

Keynote, which monitors the performance of 35 major online retailers, flagged Sears' Web site as one experiencing major difficulties on Friday. Sears.com was mostly unavailable between 10 a.m. and 12:30 p.m. Eastern Time, and again between 1:30 p.m. and 5 p.m., White said.

Another retailer in trouble was Saks Fifth Avenue. Between 7:30 a.m. and 1 p.m. Friday the Saks home page became up to 10 times slower than normal and didn't load at all for half of its visitors, he said. Gomez, another Web monitoring company, said the availability of the Sears Web site dropped to around 61 percent Friday before recovering on Saturday.

Both Sears and Saks displayed a Web page notifying visitors of the technical problems when their sites weren't loading properly, White said.

"Sears.com is now fully functional and we continue to monitor the volumes [of traffic] and take steps to ensure the site can continue to serve our customers," a spokeswoman for Sears said via e-mail. "Friday, due to higher than anticipated peak volumes, traffic was interrupted, but Sears.com is operating fine today as it did all weekend. Please note that the site was never down per se; it experienced intermittent interruptions.

The IDG News Service is a Network World affiliate.