Answer from Kyle, PHR:

It means there’s always a risk that a terminated employee could make an issue of the termination, claiming that it was discriminatory or based on some other illegal reason. (There’s also the risk that they will bad-mouth your organization on the internet.)

In most cases, employers can terminate an employee for any reason or no reason at all. However, there are a handful of illegal reasons for firing someone, and nothing you do can guarantee that an employee won’t call a lawyer or the Department of Labor and claim that their termination was for one of these illegal reasons, even if you had great documentation and they’d done something completely worthy of termination. This is why no termination is risk-free.

That said, while you can’t eliminate all risk, you can and should do everything in your power to reduce it (e.g., having a clear, well-documented reason for the termination, being respectful to the departing employee). If you have a good reason for the termination, and the terminated employee knows you can provide evidence of this reason, they’re much less likely to try to bring a claim against you.