When it comes to weights and lifting them, often people slack off or just forget about form because it's easy to do so. Additionally, it gets a little hard to think about sitting up straighter and rolling your shoulders down when you have a 100 pound weight above your head.
Often people are not doing the exercise they think they are because of poor form. As such, weight lifters will commonly do a bench press when they think they're doing an incline press, or a military press when they think they're doing a bench press. People fail to realize that which exercise you are doing isn't determined by the bench you use, but how you sit on it!