Not balancing abdominal work with back work. Most exercisers spend so much time working on their stomachs, they fail to adequately address the opposing muscles of the back. The goal of any exercise routine is to provide a balanced workout that addresses all areas of the body equally. An imbalance between these muscles can lead to back pain, poor posture, poor functioning and a myriad of other orthopedic concerns. Back extension work, in which you lie face down and pull your chest off the floor, is a simple exercise in the opposite motion of a stomach crunch.
Strength work behind the head.
These exercises place the shoulder in a position that often leads to injury if performed with too much weight or when performed repeatedly over time. Each exercise you include in your routine should be examined for the risk involved versus the benefits gained. In these two cases, the math doesn't add up. These are common mistakes I have seen which can have a great impact on the effectiveness and safety of your exercise. They included not stretching enough, lifting too much weight, not warming up, not cooling down, exercising too intensely, not drinking enough water, leaning heavily on the stairstepper, not exercising intensely enough, jerking weights while lifting and consuming energy bars and drinks for moderate workouts.