Bad Weight Lifting Techniques and Habits
A lot of people fall into bad habits when they are weight training. There are a lot of issues that can arise, so my goal here is to only cover the major ones - that is, issues that will have a significant effect on your performance and muscle gain.
Incorrect exercise form is probably the most common problem there is. The main issue that a lot of people have is cheating, but some just do exercises completely wrong and end up injuring themselves. The best way to avoid this is to look over diagrams and videos or ask an experienced lifter for help with your technique. Some of the most common exercises that people do flat out wrong are the squat, deadlift, and seated rows. When doing squats, people will often incorrectly bend their backs, which'll greatly increase the risk of injury - the same goes for deadlifts. For rows, a lot of people seem to have an issue with leaning forward; your back is supposed to stay straight, as you should only be moving the cable with your arms. You'll be using the muscles in your back, but actually bending your back will cause injury and decrease the intensity.
Cheat reps are another thing I see people doing a lot in the gym. These are okay every once in a while, but you have to realize that you're not getting a full workout like that. You may actually be causing some muscular imbalance. The two most common exercises in which people cheat are standing curls (dumbbell or barbell) and the bench press. When doing curls, people tend to lean backward, granting themselves additional leverage with which they can move the weight. This is a good way to hurt your back or just not work your arms at all. You should not be moving your back; in fact, only your forearms should be moving. And with the bench press, I notice that a lot of people tend to only do half or quarter reps. Again, this can be useful sometimes, but it's generally a bad idea. Many will also arch their back, lift their legs up off the floor, or bounce the weight off their chest. All of these are bad, especially the middle one, as it can cause injury in the shoulder or back.