My biggest mistake when I started out was not eating enough, and thus not fueling growth. When I finally learned how to eat, my biggest mistake was eating too much, and thus rapidly going from skinny to skinny-fat. I'm still paying the costs of that. These same two mistakes are made pretty consistently by a lot of beginners. We either don't know that we need to eat to build muscle (in my case, I grew up being told "food makes you fat," without any guidance whatsoever on how food is actually beneficial beyond vegetables have vitamins and milk has calcium), or we do know that we need to eat to build muscle and get recommended ridonculous, irresponsible nutritional programs like GOMAD. Or we just get told to consume a pile of supplements.