Not everything negative that happens to an employee is the basis for a lawsuit. Employees have to allege both that they were on the receiving end of some sort of negative feedback and that there were consequences that changed the terms and conditions of employment.
For example, a warning letter that cuts off a promotion might qualify, while the same letter with no consequences would not.
Recent case: Martricia Chapman, who is black, sued her employer for discrimination after she received a warning letter and was asked to submit to an investigative interview. But nothing else happened to her.
She sued. But the court said the letter and interview alone weren’t grounds for a lawsuit—and wouldn’t have been even if she could prove that racial discrimination was the motivation for the warning letter. (Chapman v. U.S. Postal Service, No. 11-11028, 11th Cir., 2011)