Mistakes to avoid when working at a startup include letting a small budget hold you back by thinking outside the box for cost-effective solutions, using your voice incorrectly or not at all by actively listening and adapting to others' communication styles, waiting for things to be handed to you by taking initiative and finding opportunities for growth, staying within your job description by being flexible and open to new challenges, not setting boundaries which can lead to burnout, being inflexible when faced with change, being wary of feedback but learning to receive it constructively, over-communicating effectively to avoid duplicate work, and giving in to imposter syndrome but recognizing your qualifications and worth. By avoiding these common pitfalls, you can thrive in a startup environment and set yourself up for success.