That you don't know it all.
When you reach a point where you think you know it all, you're in trouble. Your growth stops. You repeat mistakes. You refuse to change what isn't working. You become arrogant and unteachable.
People are the most susceptible to this when they are a young leader or a veteran leader. When you're young, you are ready to conquer the world. You have all the answers. When you are a veteran, you can develop a "been there, done that, know that all ready" attitude.
The most effective leaders are very aware that they still have much to learn and lots of room to grow.
