If private company information was unintentionally (by pure accident)disclosed by an employee of the same company, is the employee subject to penalty?

You haven't defined the (termination, liability, criminal charges, etc), so I'll run out the possibilities. The employee could be demoted or fired. Most employment is at-will and does not require cause for demotion or firing. The employee could be held liable for the financial damage. If you aren't paying attention and hit me with your car, a pure accident, you could be liable to me for the injury your accident caused. If a surgeon slips during surgery, a pure accident, the surgeon could be liable to the patient for injury caused. Similarly, an employer could sue the employee for damage caused by the employee's accident. The employee is unlikely to be criminally liable (either by fine or imprisonment) absent extenuating circumstances (high security clearance position, facts calling intention into question, court order protecting the secrecy of the information, etc.). Good luck, Todd