Retail is (still) Detail for eCommerce Websites
"Retail is detail" has been a favorite refrain of marketing 101 teachers for decades now. While you may have imagined the concept as it applies to a physical store (picture a grocery store stock-boy arranging and aligning products so that they are perfectly presented to the customer), attention to detail is just as important, and arguably more so, when it comes to ecommerce websites.
Consider this key difference between physical retail and online retail: At a physical location, a shopper has already expended some of their time and energy just to visit you in-person. At this point small mistakes they encounter in-store may irritate or annoy them; it happens. But, as long as these annoyances remain below their personal threshold they are unlikely to actually leave your store. In this case leaving would require even more of their time and energy to travel to your competitor's location. So small problems can often pass without significant effect.
The crux of eCommerce websites
In contrast, an online shopper needs to expend a minimal amount of their time and energy to visit your website. Any miniscule issue encountered on your site may be irritating enough to motivate shoppers to navigate away from your store as quickly as they can click a mouse button. The sunk-costs that make it harder for a customer to leave a physical store simply do not apply online.
For a few examples of annoying (and possibly customer-alienating) mistakes made on some well-known ecommerce sites, check out this article: 10 Surprising Mistakes on Large Ecommerce Sites
As you can see, even the big names in ecommerce are not immune from the occasional oversight, but the key to retaining customers is learning from your mistakes and ensuring that they do not happen again. If possible, regularly have several different people read through and test the functionality of every page on your site. It's always a good idea to run your site through what's called the "grandma test". Imagine a grandmother actually using your site; are the product descriptions clear to her?, can she easily navigate between pages and through the checkout process?, is the payment and shipping information straightforward?, etc. Your goal is to identify and fix anything that could frustrate your customers before they ever experience it.