1. No communication - there are still companies who make that sin. The right way goes like this - when something goes wrong in the company and it can prevent your customers from using your service or product for more than 5 minutes (although it depends), the company must find a way to warn the customers about the problem and provide estimated time in which the problem will be resolved. Problems happen all the time, but informing your customers is the first thing you need to do.
2. Ignoring customers' needs - creating a product or providing a service is a great amount of labor, but things don't stop there. Adapting is one of the keys to success, one of the keys to being adaptive is to meet your customers' needs. Whether they'd be for better customer service or making small changes to your product. Try to collect customer feedback and find out what your clients really need, so they can keep working with you.
3. Ignoring social media - we've said it before, social media is already a powerful marketing tool, mandatory if you want to make something of your business. People there talk. They talk a lot. Like it or not, sometimes they talk about your company. If you ignore the conversation things will get ugly. Keep in touch with the social networks, it is the best way to find what your customers think about your company.
4. Failing to deliver - when you promise something to a customer - make it happen. When your clients contact you or your customer service, they expect a solution to their problem, if you can't find one - you are in trouble. So, if you can't find a solution right away, you will promise to find one as soon as possible (or you can say "I am sorry, but I don't think I can help you" and suffer in the customer service's hell...) and then find it. Remember, you don't need to deliver, you need to overdeliver.
5. Use computers instead of humans (where humans are needed) - this comes to the situation where you have made a mistake. Nobody's perfect, so when it happens you need to apologize. Not with a computer generated message (if you have a computer generated apologizing message... wow...), but in personal.