Some people will just come back and order later.

It depends, because failures can lead to mis-attribution of the problem. Say any of Internet Explorer, Modem, ISP, DNS, etc. fail while a customer is trying to access Amazon.com but, by chance, the failure is over when they access Barnesandnoble.com. In this case, the person will probably think something is wrong with Amazon.com, when that isn't true (computers really are black boxes to nearly everyone).