Wal-Mart kicked off its Black Friday sales online at midnight Thursday, and soon after some customers took to social media, claiming they couldn't stock up on exactly what they were looking for.
Business Insider first reported on the shortages.
One user's Facebook comment reads: "Just was on your website looking at clothing. How in the world do you expect to have a good Black Friday or Cyber Monday when everything is out of stock. Really."
Another said on Facebook: "Did the east coast end up with all of these items already sold out. So very disappointed."
One woman also wrote: "At 2am Thursday morning you were out of some of the items already! Thought I would look this morning...sigh."
A Wal-Mart spokeswoman told CNBC: "Black Friday is about offering our customers some of the best prices on the hottest items. Even with significant inventory, hot items on Black Friday sell quickly online."
Wal-Mart's Cyber Monday deals will roll out online at midnight, come Monday.
"Every day of the holiday season is important," Scott Hilton, chief revenue officer of Wal-Mart's e-commerce division said in a statement earlier Thursday evening. "But, there is one day that gets our customers (and us!) really hyped up…Cyber Monday."
As traditionally offline retailers fight to keep pace with internet giant Amazon this holiday season, it's crucial that they're able to meet consumers' demands at peak hours and fulfill online orders.
Amazon's Black Friday deals this year include markdowns on many of its Alexa-enabled devices, along with TVs, toys, sporting goods and home furnishings.
Wal-Mart, though, has been touting heavy investments in its online business. On a recent call with members of the media, management said the company was well stocked ahead of the holidays and wasn't concerned about running low on items.
Starting Sunday evening, Target.com will feature "deep discounts" that run through Saturday, Dec. 2. On Monday, Target's entire website will be marked down by 15 percent.
As of 5 p.m. ET Thursday, $1.52 billion had already been spent online, according to a survey by Adobe Analytics. That represents nearly 17 percent growth compared with a year ago.
Of note, a record 46 percent of total traffic to retailers' websites was stemming from smartphones, Adobe found.
Despite the activity online, throngs of shoppers were still flocking to stores.
"There's exceptional traffic," Craig Johnson, president of Customer Growth Partners, told CNBC Friday morning. "It's definitely up."