Same as most. Wish I new that if I had stuck with a basic program and tried to lift bigger each session I would have improved my strength and overall fitness.
Also the relationship between food, rest and exercise and how best to use them together to achieve the best body shape.
I look back when I was young and followed numerous programs, from numerous people who all thought they new everything about body building/weight lifting. I tried so many different things to lose weight and gain muscle. I achieved great fitness but little change in body shape or strength. I wish I simply built up my squat, bench, deadlift and press and I would have achieved so much more as I'm starting to find out.