Amazon.com Inc.’s Prime Day 2018 debuted with error messages and broken landing pages.

The 36-hour sale started at 3 p.m. Eastern Standard Time July 16 and will run through July 17. Many shoppers received error messages both on the site and in the app while shopping for Prime Day deals.

For example, the “Shop All Deals” button led shoppers to a loop of Prime Day landing pages without showing shoppers any products. Other shoppers received errors while trying to checkout and had to try more than five times to successfully purchase products. Other shoppers report not being able to add products to their cart, as well as having previous items erased from their cart.

More than an hour into Prime Day, many of the category landing pages for the deals are still displaying error pages or no items at all.

An Amazon spokesperson says it is “working to resolve this issue quickly.” Still, Amazon says that customers have ordered more in the first hour of Prime Day 2018 in the U.S. compared with the first hour of Prime Day 2017.

Amazon’s initial problems started shortly after the sale went live at 3 p.m. EST and lasted for nearly two hours until 4:50 p.m. EST, according to performance analytics firm Catchpoint Systems Inc. Although, shoppers still received error messages in Amazon’s app as late as 8 p.m. EST.

“Even though the Amazon site/homepage was accessible to users, in some instances, product search was not working, with users receiving an error message,” says a Catchpoint spokesman. “This lasted from approximately 4:15– 10 p.m. EST and impacted users nationwide.”

Last year, Amazon.com’s traffic increased 49.1% on Prime Day 2017 over a comparable Tuesday two weeks earlier, according to SimilarWeb data. However, Amazon.com did not have widely reported issues last Prime Day.

Amazon’s issues today show that not even the largest web giant is immune to having its infrastructure overwhelmed by high demand, says Sumit Dhawan, CEO at cloud computing vendor Instart Logic Inc.

“Online retailers are used to being ready for three or four times their normal daily volume during peak holiday periods,” Dhawan says. “Today’s meltdown at Amazon shows that online retailers need to be ready for literally 100 times their normal volume during major promotional events such as flash sale.”

Amazon is No. 1 in the Internet Retailer 2018 Top 500.