First mistake always seems to be the lack of confidence in their new found role, and rightfully so however everyone can see that and in some cases there is one or two that will push the limits to see what they can get away with. Just that same as kids do with their parents. The trick is to show everyone that you can handle the pressure, and for those that want to push the limit one must take care of those situations ASAP, sit the person/people down in a private office setting and talk with them, make sure they understand that certain behaviour will not be tolerated.
Second mistake usually involves organization or lack of, this leads to mistakes, overruns, extra costs to name a few consequences. This usually occurs due to ones lack of confidence but it is easily corrected simply buy taking a little extra time each day to go over your tasks, placing them in a priority sequence and coming up with a contingency plan, you can honestly do this while having your first two cups of coffee in the morning before heading to the office. This simple procedure helps to define clearly your daily goals, keeping everything running smooth and allowing for the day to be less stressful.
Third and in most cases the simplest to do and hardest to balance: the dreaded personal limitations, most new managers want to make a good impression, unfortunately they tend to do this by taking on more than they can actually handle, first they are new to the position and need some time to get through the dreaded learning curve, however they want to make a good impression and hit the ground running. Some get lucky and can do so without any issues, but for the majority all that they accomplish is causing chaos within the organization and head aces or sometimes health issues for themselves. A new manager should learn about the organization, talk to some co-workers find out their about how things have worked in the past, what are the inside company procedures, heck even ask them for input to improve company functionality or moral. The more a manager knows about the people and procedures within the organization the easier her/his job becomes.
Managers roles are diverse and cover a wide range of duties but one thing that every manager needs to balance is delegation, make sure to spread the work load as needed but not just to anyone the person/people you delegate a task to must be capable to complete the job at hand.