I probably still have a lot of mistakes, but in hindsight....
Trying to lose fat while trying to build muscle, but not knowing WHAT to eat and what not to eat. Avoided all fat, but continued to eat truckloads of carbs and fruit. Basically meant not eating nearly enough.
Also, completely trusting a single book... the Matrix Principle.... which is about light weight, high reps (half reps and such) and short rest times.... I thought everything in that book was the truth. Yeesh.