Be sure your staff is trained to notify you (or your manager) if a customer has a complaint regarding the product or the service. They should also, notify you if a customer becomes difficult or gets out of hand'. The most senior person in the store should handle these situations. They will do a better job and others will learn by watching. This could involve replacing a product or offering a free coffee coupon or giving the customer their money back. Please train your staff to never, never, raise their voice to a customer or to become involved in an argument with a customer. Never touch a customer in a confrontational moment. Never handle a situation with force. Listening to a customer will work wonders. Be aware of your other patrons who probably will be watching how you handle the situation.
If any sort of personal problems occur with your employees or between your employees that may affect the work environment, it is best if they know that they can talk to you. Try to resolve the problem compassionately yet efficiently. Common sense and good judgement is needed in these instances.