Hiring wrongly
Don't be afraid of making bold advances. Hire whoever you deem appropriate. Help your preferred one in reaching the required performance level. Spend personal time on coaching, don't rely on the company's onboarding program. Let colleagues know that you support the newhire and expect them to do so. If turned out to be mistaken, correct immediately and ruthlessly. Don't wait for others (your boss) recognize your wrongdoing. Apologize and go on.