One of the biggest things that comes to mind is the ego lifting. I see a lot of guys doing half reps or forced reps with weights that are simply too heavy for them but they do them because they think it makes them look cool.
If you are not getting full range of motion on a lift, you are doing it wrong. If you are having a spotter force more than half of your reps you're using weight that is too heavy. Drop the weight, perfect your form and focus on full ROM.
Another big one is not pushing to true failure or close to failure. It's no mystery that hypertrophy and mTOR signaling is triggered by stimulating the muscle. The greatest stimulus happens when you approach failure within 5 reps. The problem with this is that most people don't know what their true failure point is and end up quitting the lift long before they've reached 5 RIR (reps in reserve). They quit when the lift gets a little difficult but that's the point where you need to push through. This is where you're going to achieve the greatest stimulus. Maximal recruitment of muscle fibers is the goal and this happens when you begin to feel the burn.
Junk volume is another big one. Guys think the longer they spend in the gym the better. Not so. Once you've stressed the muscle to the point of mechanical failure, you're basically done for the day. The muscle got the message to grow. It's time to go feed it and relax. By adding more exercises you might see a little bit more growth but at a certain point the curve flattens by a great deal. Now, if you're an advanced lifter and competitive, I won't tell you not to do 18-30 sets per muscle per week (I'm guilty) but as a beginner or even intermediate it's just not necessary and possibly counter-productive. You're just accumulating unnecessary fatigue and risking injury.
That's just some stuff off the top of my head.