You can't fix what you don't trackGoogle AnalyticsYahoo Store
If you have a technical glitch in your checkout that prevents your customers from placing an order, would they tell you? Perhaps if you are lucky some visitors will tell you, but most likely they will just give up and leave your site, a classic case of "unknown unknowns": You don't even know that you have a problem so you can't fix it.
This is what happened to one of our clients recently where a glitchy implementation of a third-party script prevented some shoppers from placing orders, resulting in significant lost revenue. Fortunately we had implemented checkout error tracking, which sends error messages as events to Google Analytics. This helped us quickly pinpoint and fix the issue.
Obviously you need form validation on the checkout so that orders are only submitted with valid information. You'd want to show a friendly error message to shoppers so that they can fix any typos and this would not typically result in checkout abandonment:
Most errors have a "normal" abandonmnent rate, but this one...Ouch!
Finally, even in this case, it still takes an analyst or a motivated store owner to actually look at the data and use it. Beware of black-box tagging implementations that slap the same tracking code on every page and treat everyone the same. Data quality is earned.