This has got to be the single most frequent training mistake I observe in gyms:
Sound: Elvis Presley, Trouble (my 2002 Posing Music)
One of the keys to bodybuilding longevity is injury prevention.
In the late 90's I suffered from a rotator cuff injury that took me more than six months to cure.
FACT: The shoulder is the most mobile joint in your body, but with this mechanical freedom comes vulnerability. Your shoulder joint is not designed to lift weights behind the plane of your body.
TIP: Avoid behind-the-neck presses, pulldowns or pullups, and keep your hands in front where you can see them.
FACT: The shoulder has a delicate inner sleeve of muscles called the rotator cuff. Because this inner layer is barely visible it is often neglected at the gym. This is a big mistake - as I personally found out the hard way. A weak rotator cuff is easily injured, causing pain and disability.
TIP: Begin every shoulder workout with some rotator cuff exercise as part of your warmup. I have been doing so ever since and have never experienced shoulder problems again.
Train hard, but, above all, always train smart!