A close friend and martial brother of mine (we tested for our instructorship together) got a "broken neck" in 2000 and has, basically, been out of training since then. He does train some, but has to take things very carefully.
His injury showed up during training, but wasn't caused by training as far as we know. He was performing a technique on someone and his neck locked up. Logically, what he was doing at the time
couldn't have injured his neck. Specifically, he was doing what we call a "puter kepala" in Silat. Aikido calls it a "kaiten nage." I think Judo calls it the same. But my friend was the guy doing the throwing, not the one being thrown. So we're reasonably sure that that moment's not actually when the injury occurred.
When he went to have it looked at and x-rayed, they found that 2 of his vertebrae were fractured. One of them had already calcified some ... which meant an older injury. The other was fresh. What we
think happened (but have no way of knowing for sure) is that the calcified vertebrae was fractured in a car wreck he'd had in 1999. It had healed some, but the other vertebrae either had a hairline fracture in it, or was just weakened. We think that something else actually fractured it, but we don't know what, and that when he was doing the throw, the fractured segments shifted.
Fortunately, there was no damage to his spinal cord, but he was laid up completely for several months.
However, another guy that I know (a chiropractor by profession) got his neck broken in a motorcycle accident. He said that the silver lining was that when he returned to training/teaching, that he
had to learn to do things without using brute strength. He became very soft in his material and had to really delve into his understanding. He said that, in the long run, he thinks the experience really helped his martial arts ... but he wouldn't advise others to take that route
Mike