What kind of injury can one sustain from improper weight lifting?

You can do a lot to yourself by improperly training. Most of the injuries are muscle tears that heal easily enough, but some are not so pleasant. You can do some serious damage to your back, including spinal injuries. It's also pretty easy to mess up your shoulder through impingement and rotating it in ways it would rather not. Finally, you can really easily hurt yourself if, for example, you're trying to squat too much weight in poor form, and you fall over with a couple hundred pounds on your back.