If you're the world's largest online retailer, and one of the most valuable companies on earth, the worst thing that could happen would be if your customers were unable to add items to their cart during one of your biggest sales of the year. Of course, if it happened two years in a row, people start to wonder how that's even possible. 

This morning, for about two hours during Amazon's Prime Day sale, customers took to social media to report that they were unable to add items to their cart as they tried to scoop up deals on gadgets and other deeply discounted items.

That's sort of a basic function of an online store, and you can imagine their frustration after all of the hype surrounding Prime Day. You can especially relate when you remember that Amazon had a similar outage on last year's sale. An Amazon spokeswoman declined to comment.

This is actually a really big deal. Analysts estimate that Amazon will bring in as much as $5 billion over the 48-hour sale that started this morning. That would make it Amazon's biggest day ever. A two-hour glitch during prime shopping hours the first morning translates into enough pain that the company's stock had dropped as much as 0.5 percent.

That might not seem like much, but for a company the size of Amazon, that translates into real money. And while it's too early to tell what the actual financial impact of those lost sales might be, it's not hard to imagine that it's easily in the millions of dollars.

You had one job.

The bigger problem for Amazon is that it looks unprepared, for the second year in a row, for the surge of shoppers that it invited to shop Prime Day. It's an especially glaring issue since the shoppers who participate have already ponied up a fee to be a part of the company's exclusive Amazon Prime membership.

It would be like inviting all of your friends over for a huge summer BBQ, and discovering that your grill won't light. I mean, stuff happens, but maybe that's something we should have checked ahead of time. 

Actually, it would be like selling tickets to a huge summer BBQ, to all your friends, and then discovering the grill won't light. 

That's really bad. 

Stuff happens, but that's not an excuse.

Look, stuff happens. That's business. People understand that sometimes technology doesn't do what you want it to. And if you run a business, you've surely had an experience where things just didn't go the way you planned.

But when you're Amazon, there's an expectation that you're going to have your stuff together.

You're the experts. You're the guys who do this for a living. 

And what about the thousands of small business merchants who participate in Prime Day by selling their own products through Amazon's Marketplace and Handmade storefront. 

Here's the lesson: when you make a promise to your customers, to your partners, or to anyone, your job is to keep that promise. When you promise low prices on lots of cool stuff, you break that promise when people can't actually, you know, buy any of it. 

When you promise other businesses access to millions of customers for their products, you break that promise when they can't sell it because you're having a "glitch." Your job is to do whatever it takes to deliver on that promise--especially when your track record is beginning to lean in the direction of unprepared. 

It's not unheard of for websites to malfunction, but I think you could argue Amazon should have been prepared considering this isn't the first time they've experienced problems on Prime Day. It's not like they didn't know that millions of people were about to descend on their site.

Plus, it's not clear that increased traffic had anything to do with this particular shopping cart glitch. Regardless, whether it was the increase in traffic or a rogue piece of code that wasn't behaving, it's still your job to get it right when you've made a promise. 

Amazon still hasn't told customers what happened, but the site seemed to return to normal around 1:30 pm EST.

Correction: An earlier version of this article mischaracterized the Amazon Prime Day customer problems reported on social media Monday as an outage connected to its cloud technology. They were related to a shopping cart malfunction.