A more thorough explanation:

The after-acquired-evidence doctrine is a rule in employment law that states if an employer fires an employee for an illegal reason and later discovers evidence of misconduct that would have justified a legal termination, the employee cannot be reinstated.

For example, if an employer fires an employee for reporting sexual harassment, which is illegal, but later discovers that the employee stole company property, which is a valid reason for termination, the after-acquired-evidence doctrine would prevent the employee from being reinstated.

The doctrine is used to protect employers from liability or limit the available relief when they learn, after an employee has been terminated, that the employee engaged in wrongdoing that would have resulted in a discharge anyway.

An example of the after-acquired-evidence doctrine in action is the case of McKennon v. Nashville Banner Publ'g Co. In this case, the Supreme Court ruled that an employer could use evidence of an employee's misconduct that was discovered after the employee was fired to limit the damages awarded to the employee in a wrongful termination lawsuit.