According to this post from one of the Branham & Day blogs, the Tennessee Supreme Court held in Teter v. Republic Parking System, Inc., that after-acquired evidence can be presented as a defense to the merits of a claim for breach of an employment contract, but the burden of proof is on the employer.
What this means is that where the employee can only be fired "for cause," even if the cause the employer had in mind is insufficient or can't be proven, the employer can take discovery to find some other cause that it didn't even know about at the time of the dismissal.
I have argued for something similar under Virginia law, citing some old cases. At one hearing, the other lawyer argued, oh, those cases are old. The judge asked whether counsel believed that the age of enlightenment had reached the Supreme Court of Virginia only some time after those cases were decided.