If there is one muscle group that identifies you as someone who practices bodybuilding is the arms. When someone asks you to flex a muscle, more than likely the muscle flexed will the the biceps of the arms. Isn't it true?
Thus, for many beginners who are just getting started, getting big arms is a huge priority. Now, keep in mind that symmetry is key and you don't want to have a situation where the only muscular part of your body is your arms. Thus make sure that you place as much emphasis on every single other body part (should go without saying).
With that said, I feel that there are two mistakes that are holding a lot of people from achieving the sort of arm growth that they could get:
