Many people train the body incorrectly in many ways. One of the biggest ways is the simple failure to train the muscle groups during a workout in the proper order. Many people train arms before back, shoulders before legs, chest before legs, and a wide array of other combinations that do not put the bigger muscle groups first.
Always start with your largest muscle groups first. These require the most energy and carbohydrate output to train. In addition, the larger muscle groups do not inhibit performance when it comes to the smaller muscles after (for example back to biceps), whereas working the smaller muscles first can inhibit optimum performance when doing larger muscle groups after (biceps to back).
This can apply to a full body circuit as well as just a two to three muscle group workout. The order which you train the muscles should always fall with the largest muscle group first to do it properly. The last thing you want to do is exhaust a "synergist" prior to a protagonist. A synergist is the helper muscle to a main muscle. These are usually smaller muscles compared tot he larger ones. Exhausting synergists prior to training a large muscle group will lead to poor performance on the large muscle group due to the inability of the synergist to assist in the movement. Think doing biceps before doing pull ups, for example.
By training the body in the right way, you will always ensure optimum results, prevent injury, and achieve the greatest quality strength and aesthetic gains.