1. Thinking the customer is always right.
This will kill your margins. Reducing costs, giving away freebies, comping things.... all of these practices will put you out of business. The customer isn't always right. While you want to try and smooth things over with an annoyed client, you don't want to destroy your business in the duration. Think about how to resolve a problem with value added services, not discounts or freebies.
2. Toxic employees.
This should seem obvious but toxic employees can cost you customers and turn off great employees. Bad HR practices will hurt your company. Even poorly behaved contractors can ruin your company. Avoid this problem all together by looking beyond experience and more at attitude. Remember, external customer service starts with internal customer service.
Don't let these two things ruin your company or potential growth. Know that the customer isn't always right and that your employees' bad behavior will hurt your profit margin.