There is a situation when you may have a legal obligation to provide information about a former employee to a prospective employer. Assume that you know that a former employee has a history of criminal violence or extremely aggressive behavior. Another employer approaches you for a reference in connection with a job that would have your former employee working closely with members of the public. Must you disclose what you know about the employee's past conduct? What if you're not sure that the information is true? Your risk in remaining silent is that you could be sued for negligently failing to disclose the information if the former employee were to subsequently harm someone while on the job. On the other hand, you could be sued for defamation if you do disclose the information and it turns out not to be true.
Faced with this type of situation, you should consult an attorney to determine what, if anything, you may be obligated to disclose.