Here are some exercises that most of you know you should be avoiding but you don't really know why you should be. I don't personally believe in saying "never" do exercise A, or exercise B but the following are just some exercises you should avoid or find a better alternative for. Remember almost every exercise has a potential risk to it, so just do the exercises with the greater reward relative to its risk.
1) Upright row-This is a fairly common exercise you see people doing in every gym across America to develop/strengthen their shoulder. The issue with this exercise has to do with that very muscle, the shoulder and the shoulder joint. Upright rows can cause shoulder impingement over time for many people especially if done with a narrow grip. If you are going to do this exercise try and switch to a wider grip.
OTHER ALTERNATIVES: Military press, dumbbell press
2) Behind the neck pulldowns-Pull downs to the front of the body are a great exercise, this variation not so much. Just like the upright row this exercise can cause issues with the shoulder and shoulder joint. Also, since most people lack good shoulder flexibility, they will tend to compensate by bending their neck forward and causing issues with the neck and C spine.
OTHER ALTERNATIVES: Normal lat pulldown, Pull ups
3) Behind the neck shoulder press-Are you noticing a trend here? Most of these exercise have to do with a movement in which the weight is going behind the head. This exercise has the issues as the above exercises. Shoulder problems and rotator cuff issues can arise as well as problems with your neck.
OTHER ALTERNATIVES: Shoulder press in front of the head, military press
4) Stiff leg deadlift with legs elevated-The stiff leg deadlift, done right, actually is a great exercise and should be done to improve the hamstrings, back and glutes. However, when you try and get more extreme range of motion by elevating your legs off the ground and standing on a box or step you can cause issues. What tends to happen with this approach is once you get to a certain depth on the lift you lose the arch in your back. By losing the arch in your back you shift the tension from the musculature of the back to the connective tissue and bones of the spinal column. End result is you increase your risk to hurt the low back.
OTHER ALTERNATIVES: Regular deadlift, hamstring curl
5) The basic sit up-If you like having a healthy low back then I would avoid/minimize doing floor sit ups. This old exercise ends up doing more harm than good. This exercise causes you to flex at the spine over and over and eventually can lead to damage to your discs and low back. Now I do think here and there you should toss in some sit ups because the last thing you want to lose is the ability to sit up from a bench press after pressing 300 plus pounds, that would not be good. But for the majority of your time I would use other exercises that have a higher reward than risk.
OTHER ALTERNATIVES: Planks, side planks, med ball chops and lifts