The biggest career mistake I have made is to assume that there is something called career mistake.
Let me explain. When we use the word "career mistake", we are referring to an incident in our lives when we took a certain decision that turned out to be "wrong" later. It may be that we rejected a job offer with a startup that turned out to be a "Google" and we missed being a millionaire.
We think that such a decision was a mistake and deal with it in multiple dysfunctional ways. Some people just ruminate about the incident throughout their lives and miss on the later part of their life living in the shadow of that memory. Some people try and avoid that mistake and make even bigger mistakes. For example, someone who missed joining a hot startup would work for many failed startups later or worse still bet all his/her money on founding a startup.
When one looks at the so called "mistake", one realizes that it was the best decision with the information and the emotional maturity one had at that point in time. What one has to work on is to improve the informational content and the emotional maturity using all the feedback life gives us. Yes feedback life gives us is what can be called as a mistake. One just takes it as what it is, a feedback about our knowledge and emotional maturity.
So the biggest mistake would be to assume their is something called a mistake. There are no mistakes, only feedback from life.
(Please contact me to ask your question about startups, decision making, productivity, career management and happiness.)