(AMZN -1.11%) has warned customers that orders for non-essential items may be delayed beyond the two-day delivery guarantee Prime members are accustomed to. Because it is prioritizing the stocking and delivery of certain items, other orders could experience delays of as long as a month.

Some third-party retailers continue to offer speedy delivery on many of these non-essential items, sometimes even selling them for the same or lower prices. But customers won't know of them -- because Amazon is hiding these listings from view.

Amazon driver delivering a package

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Putting Amazon first

According to Recode, it discovered that for a broad range of products, from sporting goods to office equipment, on which Amazon has slowed delivery, the e-commerce giant's algorithm is highlighting its own products instead of those from third parties that consumers could get quicker or more cheaply.

Amazon says hiding the listings was inadvertent. In a statement to Recode, the retailer said it made changes to its store to prioritize certain products, and "some of these changes have resulted in an error which, in some cases, resulted in an unintended variation in how we select which offers to feature." It promised to correct the situation as quickly as possible. 

Amazon normally favors delivery of items it sells or that third-party sellers ship to its warehouses for fulfillment, because it typically can get them to customers faster than if they ship from third-party facilities. However, the algorithm apparently hasn't been updated to recognize that this may no longer be the best option.

To get around the quirk in the system, consumers looking for a specific item can look on the listing for a text box that says "New & used" offerings from other sellers. When clicked, it will show all the price and shipment dates available.