Most accidents are the result of complacency, as Jackie said. Sometimes though, things happen which are totally unexpected. Mostly, the danger is seen in retrospect.
People talk about never wearing long sleeves while running a lathe but short sleeves are often not an option. You want to try working in a heavy machine shop when it's 30 below and the doors are open. In such conditions you wear coveralls, gloves and whatever else you need to stay warm. Even if you are cold, you still have to do the job.
I once saw a heavy canvas parka ripped right off a guy. If he hadn't been such a big, strong fellow, he might have been flapping around the roller instead of just his coat. As it was, his arm was badly broken but at least it was still attached.
I've watched my own hand travel slowly around a big sprocket; able to do nothing but wait until it made the curcuit. I was some perturbed when I realized the sprocket ran through a slot in a piece of plate. The hand couldn't go all the way around. I like to recall how I stood there stoically while I waited for someone to notice me but, in reality, I probably shrieked like a frightened girl! I remember the feeling of disbelief that I could have gotten caught that way! I was way too smart for that. Sometimes, things happen. Regards, Bill.