Five years ago I handled a situation unprofessionally and received a letter of reprimand.

Five years ago I handled a situation unprofessionally and received a letter of reprimand. I did not follow a direct order; there were things going on that were not right such as bullying but I handled the situation badly. I continued to work for the agency an additional four years and there were no other adverse actions. In fact, I received an excellent annual job performance review just after the reprimand - it doesn't make sense but they still gave me an excellent rating. I moved on to another job four years later, and am graduating with my MBA, but some applications ask if there was ever any reprimands or misconduct. How do I explain this to a future employer? I'm not proud of it and thankful that it didn't lead to worse, but it is there.