You can't do anything to protect them, other than (if you remember who they are) let them know to potentially keep an eye out for identity fraud due to the way their SSNs were handled. There is no punishment for an employer accidently giving out a paper to an employee with another employee's SSN on it. The law is not as draconian as to punish companies for minor (e.g. one person) slips in document handling (and there are not nearly enough recourse in government agencies to look into or take action about cases like this).