Mistakes happen. The best thing you can do as a manager and leader is to help your employee learn from his (or her) mistake.
If your employee is afraid of ever making a mistake, he will be paralyzed from taking action or taking even calculated risks. If he knows that mistakes happen in the course of doing business and that one learns from making mistakes, you will have a more productive employee.
Most important, be sure your employee knows that if he makes a mistake, he should let you know as soon as possible.
As soon as he does, quickly rectify the situation.
Then, discuss with him how the mistake happened. Find out what he did or didn’t do. Ask him what he thinks he can do in the future to avoid the mistake from happening again. Chances are he has already figured this out. If not, teach him what he needs to do differently to avoid the mistake from reoccurring.
Finally, you may discover that the mistake happened because policies, procedures or your assignment instructions were confusing or unclear. Learn from that discovery and decide what you can do differently as the manager to help your employees avoid future mistakes.