One that you must guard against at all times. A user might bond with one of your competitors and then use that data to carry out investigations or subversive attacks. For example, if an evil customer talks with a good customer, the evil customer can find out all kinds of things. Your plans, your prices, your ideas. Even usernames and passwords.
Humans are weak.
Also, don't ever assume that any data on your server is secure. Basically, if any customer or person can reach it, it is fair game. If you have truly proprietary information, keep it off the web server. That is, a disgruntled employee is more likely to steal you blind than your competitors. And, if they are smart and well connected with their fellow employees, they can really do damage.