Mistake : Not reading between the lines
One of the mistakes I made in my IT career was that I did not read between the lines most often. Let me give an example.
In one of situations, when I was very much into analyzing data of our department, I was asked by my manager to start analyzing data of another department which did not fall under my manager's remit. I wasn't able to understand the intention of my manager. I wasn't in a position to ask more details about the intention and neither was it made clear to me. I started guessing maybe my manager thinks that I don't have enough on my plate (whereas I had quite a lot and wasn't having enough time) or maybe my manager was not happy with my performance (whereas I was being appreciated) in the department or maybe the department (it was a separate line of business) was going to be acquired by another company and some other thoughts, and I was not happy with that. I had almost decided that I am going to quit. After a couple of weeks I learnt that my manager was going to move to the other department and head the department. I felt bad that I was thinking negatively about this, all that my manager wanted was to take me along to the bigger department. I wish I would have read between the lines correctly.