The most common mistake is the use of training complexes, following the example and advice of any "stars". Why shouldn't beginner bodybuilders do this? Because the body is not trained, just as there is no proper level of motor skills, the functional of the body is not the same as that of experienced athletes. In addition, professional athletes often use factors that speed up the process. So, more experienced bodybuilders, and even more so the "stars" of this area, are at a completely different stage of development, so the complexes and programs that they use are not suitable for a beginner athlete.
The second problem is immeasurably large, in comparison with your abilities, loads.