Amazon.com Inc said on Tuesday it resolved a glitch that made it difficult for customers to add products to their shopping carts during “Prime Day”, the online retailer’s largest shopping event ever.

The glitch, which affected Amazon’s U.S. and UK websites, sparked outrage on social media with the hashtags #PrimeDay and #PrimeDayFail trending in the United States.

Amazon said in an email to Reuters that users were facing problems adding some “Lightning Deals” products to their cart and that the issue had been resolved.

The snag was a blow to a company that takes pride in its stable technology infrastructure.

The one-day sale, for members of its $99 per year Prime subscription service, had been expected to generate up to $1-billion in sales, more than double the amount during last year’s inaugural Prime Day sale, according to Citi analysts.

Still, that pales in comparison with the more than $14-billion of total value of goods transacted during Alibaba Group Holding Ltd’s Singles’ Day shopping festival in China in November.

Amazon devices such as the Echo voice-enabled speaker, Fire TV and Fire tablet are available at big discounts as are a host of other products from high-end televisions to shoes and toys.

A prolonged disruption of its services might prevent Amazon from meeting its sales goals and could push shoppers to other retailers’ websites.

Several other U.S. retailers are already trying to cash in on the hype surrounding Amazon’s Prime Day event, by offering their own online midsummer promotions.

Wal-Mart Stores Inc launched free shipping with no minimum order threshold on all items from Monday through Friday, while Gap Inc is offering discounts of up to 75 per cent on some of its brands.

Amazon’s shares were down 0.3 per cent at $751.50 in morning trading after touching a record high of $757.34.