#5: Overdoing it. This is the "if X is good, 2X is twice as good!" approach.
"If 4 sets is good, 5 sets is better, and 10 sets must be awesome!"
"If working out 3 days a week is good, working out 6 days a week is got to be at least twice as good."
"If eating 1800 kcals a day instead of 2400 will cause weight loss, then 1000 kcals a day must work even better!"
The problem with this approach is sometimes the "good" number you started with was, in fact, optimal. By adding more sets, you may compromise recovery and train your body harder and faster than it can adapt to the exercise. If you add days, the same thing - the extra workouts might detract, not add. If you drop kcals below a your requirements to maintain, you will lost fat . . . but if you drop them too far below, your body responds by holding on to as much fat as it can and slowing down your workouts instead so you don't starve.
It can be hard to see where the optimal point is; you need patience and a way to measure. As long as you are progressing towards your goal, it's probably better to tweak a little instead of adding a lot. But the temptation to add more, more, more (or eat less, less, less) is strong. But training isn't linear, it's a bell curve - too little and you get insufficient results, too much and you get equally insufficient results, and in the middle lies the optimum work-to-reward ratio.
This one is pretty common in January, when the "I will work out every day" crowd comes in and finds they are on the far end of the bell curve, and then quits, finding the other end. Don't be like this. Find something that is getting you progress, even slow progress, and stick with it. Consistency will trump maximized work every time.