Not having a plan, avoiding the best exercises, working out too often and for too long, not training hard enough, using too many exercises with too many sets, not doing the right amount of reps for your goal, resting too long between sets, anything classified as "core training", using the gym as social hour, crappy form, bouncing the weight and using momentum, moving the weight too fast, following any workout in an infomercial, bad goal setting, working out for "toning" or "definition", not keeping a workout log, following what the "big guy" in the gym does, for men worrying about getting too muscular, for women worrying about getting too muscular, any workout that has X or CrossFit in the title, and much more!
I've often told gym members who I'm instructing, "if you ever forget the right way to do an exercise, just look around you at all the other people in the gym, do the exact opposite and you'll be just fine!" It's not your fault, it's human nature to not want to workout properly. Doing the right exercises and working out the right way hurts and it's not fun.
What I want to accomplish is to make you aware of your mistakes and teach the right way to workout whatever your goal is. It's quite simple to workout and get great results with your training. I'm going to break down each muscle group and show you what you've been doing wrong all these years, teach you the right way so you can start to see real results faster than ever.