When you confront an employee about a mistake or rule violation, she’ll probably offer some sort of quick explanation. She won’t have had time to come up with a million excuses.
If her immediate response amounts to a confession she broke a rule, it’s entirely reasonable to act on that.
If she comes up with a different story after you have terminated her, a court will look at that initial investigation to decide whether you acted on a legitimate belief that she violated the rule when you fired her. Ultimately, you don’t have to be right.
Recent case: After Tomorrow Bush pepper-sprayed an inmate in the jail where she worked, she admitted she did so without justification. When the county fired her, she sued, claiming she had acted in self-defense.
The court said her explanation came too late. The county was entitled to rely on her initial explanation when it fired her. (Bush v. Houston County, No. 10-13282, 11th Cir., 2011)