It's not uncommon to see exercises being misused in most gyms. In fact, at one point or another I have been guilty of using poor exercise selection, less than stellar form, and generally bad judgment when it comes to risk/reward of an exercise. Fortunately, I have learned (a little) through trial, error, and research, without seriously injuring myself in the process.
When deciding on an exercise, the first things you should think about are: "Will this help me reach my goals?" and "Is there a safer alternative?"
I'm not going to lecture anyone on safety. There are few exercises and even fewer things in life that won't put you at risk of some kind of injury. However, risk/reward should always be a consideration when planning your workouts.
Let me provide some common examples of poor exercise application:
Olympic-style lifts (Clean and jerk, snatch, hang clean, etc.)
The good: Olympic-style lifts allow you to develop an incredible amount of power do to their explosive nature. This is why you will see track and field athletes, mixed martial artists, and American football players preforming this style of lifting.
The bad: These lifts are very technical and potentially dangerous. Olympic weight lifters spend years learning form, developing flexibility, and train in gyms properly equipped (bumper plates, platforms, etc.) for this style of lifting, and yet still run a high risk of injury.
The ugly: There is a school of thought by some (cough Crossfit), that has clients preforming these lifts for high repetitions or while fatigued. This is dangerous and stupid. It is dangerous because when fatigued it is very difficult to perform these lifts with proper form. It's stupid because the whole idea of these lifts is to maximize power and while the conditioning effect is great, there are plenty of equally effective alternatives.
If you are going to preform Olympic lifts, spend a little time with a coach and please don't do sets of 15.
The alternative: If you're trying to develop power, try generally increasing your strength, throwing a medicine ball as hard as you can, and performing basic plyometrics (technique is important here too and there is a risk of injury, but at least you won't break your spine). If you're trying to increase conditioning better options would include: Sprinting, calisthenics circuits, low impact plyometrics like box jumps for reps, and any number of other exercises that fatigues you quickly without having you perform a technically difficult exercise.
A notable exception: I believe the dumbbell or kettlebell alternatives to these lifts to be much safer, as the overall load is lessened and it is much easier to maintain good form and balance.
