You work hard to drive traffic to your site, not to mention the money you spend on advertising and optimization. But every business will suffer from dissatisfied customers and receive customer complaints. Properly responding to the complaints is a key factor in getting people to remain loyal and not drag your good name through the mud.
So, outside of rectifying the situation, you must acknowledge complaints and let the public know that you are working to fix the problem. Blogs, viral video, email and chat rooms all have the potential to spread bad news about your company - at lightning speed. A good practice is to inform the one making the complaint that you are working to fix the situation. Then make the solution very public. Customers will give you the occasional free pass, but if they feel that your company isn't aggressively working to fix problems, they will go somwhere else.
While it may seem counterproductive to show your faults for all to see, it's better that the issue is resolved and everyone knows about it. Otherwise they will assume that it was never fixed - you will look lazy and not focused on the customer. It's the same reason that newspapers run a corrections section - developing trust.