One of the biggest mistakes I usually see in the gym is guys lifting weights that are clearly too heavy for them and are either half repping it or swinging the hell out of the weight and lifting with momentum. On the other end, you see girls lifting too light of a weight to effectively challenge their muscles. Although these criminal offenders are on the polar opposite of the weight spectrum, they both have essentially the same issue. They have chosen the wrong weight to create the appropriate amount of tension. In simple terms, how a muscle gets stronger is through a damage/repair cycle. By creating micro tears in a muscle through resistance training the body is sent a signal that it needs to repair those tears and increase the strength of the given muscle fiber. If the weight is too light, not much is going on. If the weight is too heavy, the person is probably not going through a full range of movement or cheating in some form or fashion just to move the weight and isn't placing full focus on the targeted area.
For example swinging up a bicep curl and using a lot of momentum to carry you through the movement and/or leaning back to assist you in bringing the weight up is taking a lot of focus off your bicep and instead putting more work in your back and shoulders and hips to help generate the necessary force to bring the weight up. An important thing to remember is that our muscles are blind as a bat. They have no idea what weight you are picking up, all they know is what kind of tension is being placed on them. We often get caught up with the number on a given weight (myself included) but you've got to remind yourself that what's most important is the tension. One of the best ways to really emphasize the tension being placed on the muscle is to concentrate on the negative or eccentric part of the movement. For example, when doing a dumbbell chest press lower the weight down in a slow controlled manner (2-4 seconds) to increase the time under tension for the muscle and then continue to push the weight away from your body on a 1-2 count. You will quickly feel how much more difficult a given exercise becomes and how much greater the tension being placed on the muscle is.
For guys, this can be a great way to take a step back and make sure that you are lifting a weight that you can actually handle, going through a full range of movement with that controlled eccentric tempo sometimes means going down in weight but you can sometimes get much more superior results with that increased discipline.
For women, if you are feeling stuck at a given weight or don't feel confident in taking that next jump up this same technique is a great way for you to continue to squeeze out as much value from the same weight you have been using. Remember if your goal is strong, lean muscle and you are able to do 12 reps, it's time for you to step it up in the weight department to continue to progressively overload the muscle to get stronger!
So the next time you are at the gym DON'T be that guy or girl. Now you know better! Keep lifting and working hard and results will come.