Amazon suffered glitches at the start of Prime Day on Monday, slowing sales on the e-retailer's biggest shopping day of the year.
Amazon's annual shopping holiday kicked off at 3 pm E.T. and runs for 36 hours, the longest Prime Day yet. Shoppers reported several errors — on both the desktop site and the mobile app.
Amazon did not immediately respond to requests for comment from CNBC about the glitches, but later posted on Twitter: "Some customers are having difficulty shopping, and we're working to resolve the issue quickly. Many are shopping successfully — in the first hour of Prime Day in the U.S., customers have ordered more items compared to the first hour last year."
Shares of Amazon fell off session highs immediately following the rocky start to Prime Day. The stock had gained more than 1.5 percent earlier Monday, reaching an all-time-high of $1,841.95. But it closed the day up only about half a percent at $1,822.49, and was falling about 1 percent in after-hours trading.
Some users saw an error page featuring the "dogs of Amazon" and were initially unable to enter the site. Some got caught in a loop of pages urging them to "Shop all deals." Clicking the entry link for a specific category returned the user to the first page urging them to "Shop all deals."
Some users successfully added items to their cart, only to receive an error message when trying to checkout and complete the purchase. Other users saw the "deals" page and "Shop all deals" button disappear entirely.
Prime Day was projected to break records again this year, surpassing even the "tens of millions" of Prime members who shopped the discounts last year. Wall Street expected higher sales and a higher share price for Amazon as a result.
Amazon's e-commerce sales in the U.S. are meanwhile expected to reach a staggering $258.2 billion this year, up nearly 30 percent from a year ago, according to a new survey from eMarketer that looks at the company's sales by product category.
That means Amazon is expected to capture nearly half of the U.S. e-commerce market by the end of 2018, eMarketer said. The company ended 2017 with about 44 percent of the market.
On Prime Day by itself, industry analysts had estimated the company could ring up as least $3.4 billion within a 36-hour window. This is the first Prime Day that Amazon owns grocer Whole Foods, which has its own deals running through the week.
With the glitches, however, it's possible Amazon isn't able to achieve those sales targets. Frustrated shoppers could begin to turn to rival retailers offering similar deals online this week.
"The outage is especially problematic as many of Amazon's Prime deals are promoted for a set window of time — something that could cause a great deal of frustration for potential customers," GlobalData Retail Managing Director Neil Saunders said.
WATCH: How to decide if Amazon's Prime membership is worth the price