Plan the details of your feedback and error instruction in advance. Be as specific as you can regarding the aspects of the individual's performance that could stand improvement. Say what you don't like and why you don't like it. Select a proper time and place. Although you generally will want to deliver the message as soon as possible after discovering an error or pattern or errors, show some discretion.
Private conferences are generally preferable to public humiliations.
Reward an employee who discloses his or her own errors. A supportive statement that affirms the individuals value to the operation. State the error without resorting to accusation, blame, scolding, or threats. People often feel stung by criticism. You may need to remind them repeatedly that you don't believe they are bad or stupid or incompetent simply because they made some errors.