Everyone can make mistakes in their career.
Little things in the scope of a life long journey to success might be a pain at the time but can help us grow. It's the major ones that can set you back.
Here's a few that I believe can hurt an author the most.
1. Not having a plan - If you don't know where you are going, how are you going to get there?
2. Not constantly learning new things - The world is constantly changing. As an author with a product, you need to understand and use the tools available. That requires you learn about new social media sites, new software and of course, how to write a better book.
3. Staying in touch with what readers are looking for. - Today's readers are looking for different types of book - such as shorter reads, vampires, hot reads. An author has to keep up with the market to know what people want.
4. Not Networking - again an author need to stay in touch with the world what's happen, what people want, trends, etc.
5. Growing Complacent - it is easy to keep doing what you're doing because it has always worked for you. The thing is we all need to constantly try new things to find the next new thing that works. Keying an eye open for opportunities gives us and edge.
I hope these things help you with your career as an author.