Here are my biggest mistakes...
1. Hunting without a plan. Even if it is the wrong plan, at least you put some thought into the hunt and I know that makes me more engaged when I am out there. I got caught this year not engaging the brain right away, had a buck came in chasing a doe that was one I would have loved to have killed and before I knew it they were both gone. Had I had my mind in the game, I would have had a much better chance to bag that deer
2. Not picking out trees during scouting. For me, it's great to find a bed. It's even better to have an outline of the hunt before I leave the area. Kind of like mistake number 1, it's easy to think the game is over once you find that bed, but there are a lot of variables that need to be considered. I am still learning all the variables and how to make better guesses as to how the hunt will go.
3. Not hunting enough. I know there is stuff on here about hunting smart, but I am relatively new to this, so some of this stuff I have to learn in the field. Even if it means screwing up, it never hurts to see it. And if you screw up and learn from it, well that is money in the bank in my book.
These are pretty general mistakes. If I listed all the little mistakes I commonly make, it would take a short book to cover all of those.