|No one can blame you for being upset with an employee who is a skilled “blame-thrower.” However, before focusing on her, look at yourself.|
Since it sounds like her decision-making has led to more than a few mistakes, perhaps she needs more frequent guidance and coaching from you. If you keep the lines of communication open and find that her mistakes continue, the next step is to look at your method of feedback to her.
You indicated that you tell her that she is wrong, and then she becomes defensive. Actually, her reaction is fairly normal. Rather than stating she is wrong, direct your attention to the specific decisions that she has been making and try to learn why and how she made them. As you understand more about her thinking, you will be in a better position to determine some targeted actions to prevent a repeat performance.
If her mistakes persist in spite of these kinds of steps, the time will soon come to advise her of the consequences of further sub-par performance. If she continues to make mistakes and you respond with more serious action, she will have no one to blame but herself.