You don't realise the changes that a leadership role entails
People often try to retain the relationships they enjoyed before taking on a leadership position. Leaders, especially those in supervisory roles, must be careful not to let friendships interfere with good judgment. Be aware, too, that those who know you as a colleague or peer may have a different view of you as a leader.
You mirror other leaders too closely
People new to leadership roles may try to copy a leader they respect, because the person provides an easy model. This can create a false impression of what you are really like, or, worse, make you look foolish for trying to mimic a style incompatible with your own personality. Leadership behaviours come from within. Understand what it is you respect in the other leader and think about how you can best display that attribute. If it doesn't work, don't be afraid to try a new approach.
You don't work at it
Many people hope that they have natural leadership skills, and accept leadership positions without proper training or mental adjustment. This sink-or-swim approach works sometimes, but not always! Building up leadership skills, increasing awareness of yourself, and developing a positive reputation in the organisation have much more potential for success. It will also give you more room to make mistakes without losing credibility.
You mistake public speaking for leadership
Commanding an audience is a great skill, and many leaders have it. However, it is not the sole requirement. Leaders also need to be problem-solvers and have originality, flair, confidence, self-knowledge, strong interpersonal skills, the ability to listen, vision, good organisational skills, and so on. The ability to 'talk a good fight' is not enough on its own.