Steve Bruce at HR Daily Advisor has started a series on things you don't want to say in court. An excerpt:
"I fired him for no reason"
This is the statement that "at-will believers" will have to make on the witness stand. They've always been told that if an employee is "at will" they can fire the employee for any reason or no reason. And although that's technically true, it's always unwise.
That employee is going to say he was fired because of his protected status—race, religion, national origin, sex—and your only defense is going to be "No, no, I didn't fire for that illegal reason, I fired for no reason."
No jury has believed that yet.