Where do I start lol?
Drinking 6 whey shakes a day when I was young.
Drinking aggressively 5-7 nights a week when I was younger.
Training like an asshole (swing/thrust curls, 1/4 squats, 1/2 depth bench etc.)
Eating like a bird or poorly in general.
Not starting earlier/smoking myself retarded until I was 17.
Buying any supplement but protein.
I am sure there are many more, but these come to mind.
Some aren't gym specific, but really hurt my progress. Since I have learned to eat and lift accordingly to my goals, I have made huge progress since getting back to the gym after my surgery last September.