I think there are 3 possible problems that can be associated with over confidence. Here are the following:
1. You forget the risks. When you are over confident in any aspect, you forget the risks because you are always confident that everything will go right according to what you desire. You do not mind what might be the possible consequences of your actions because you only think of the positive. For example, when playing a game of chess, you are over confident that you will win the match. But somehow, you end up losing that is because you don't think your moves thoroughly, you became over confident.
2. You become close minded. Because you are over confident, you only listen to your ideas and you will not accept any idea coming from others. You only think that the best way to approach a problem is your way without considering other solutions because you only trust yourself due to you being overconfident.
3. You hinder growth. Because of overconfidence, you do not know what things you should adjust about yourself, you always think that you are enough so you won't strive to get better. Hence, you will not be able to grow as a person because you think that you already have everything to achieve everything you desire.
Confidence is not bad as long as you learn when it is too much. Sometimes, lacking of confidence will also hinder you from getting what you desire. So I think the best way to have confidence is to learn how to use it properly. I hope my answer helps!