I did once throw out my back while deadlifting. It was completely preventable, but it turns out that the consequences were minimal. I've become a lot less worried about my back since then.

It was the day after a hard 20 mile run, and I stupidly tried to deadlift my 1 rep max in that fatigued state. Something popped and I felt a sharp pain in my lower back. It hurt really badly for about 3 days, and made it difficult to put on socks and pants. And then it healed right up. No pain ever since, and I went right back to deadlifting.