Torsion Bar

[mluder]

How common is this? It's a scary thought.

Not common at all... The torsion bard were tested to over a million cycles or some wild number like that. I just happened to be lucky.
There are a few precautions you can take. Visually inspect your bars regularly for nicks or scratches - That's typically why they fail.
Make sure the rear door hinge is not rubbing on the bar - this is a common source for the above mentioned scratches/nicks. You should be able to fit a playing card or similar between.
There's also a procedure for cleaning off surface rust. I don't know it off the top of my head so I won't even try to guess. It's here on the forum somewhere. Perhaps some one with a better memory can point you to it if a search doesn't turn it up.

All that said, your mileage may very and you could do all of this and it will still snap tomorrow. Like I said it's rare but does happen.

Cheers
Steven