It actually had nothing to do with deadlifts or bad form. I was working on my bench press and couldn't get the fifth rep up. I set the bar down on my chest and instead of calling out for help (to any of the fifty people standing nearby), I thought it would be less embarrassing to just roll it down to my knees and then stand up and set the weight down. Stupid decision. Once I got the weight to my knees, I severely underestimated my back-to-chest strength ratio and the weight plummeted to the ground, pulling my (arched) back down with it. I think it was about 215 lbs (97 kg).

The key to deadlifts is good form and not letting your back arch at all. I would recommend getting an expert trainer or an advanced lifter to help you practice your form if you are worried about it. It's worth the time to prevent an injury.