Know Your Limits
This seems like a fairly obvious thing to keep in mind but somehow the ego takes over and makes us forget our limits. There is nothing wrong with testing your strength but do so reasonable and responsibly. Ive seen many guys walk into the gym and to directly to the 120s and try to shoulder press them only to fail. The bottom line is that the shoulder has several tiny muscles in there and if they arent developed to the point of being able to realistically handle the weight you use, you could be looking at a serious injury. As good as surgeons are nowadays, rarely are shoulders restored to their previous glory.