Mine lost all of its gearbox oil through the driveshaft seals and the box ate itself; the only mechanical failure I've had in five years and 60,000 miles.
The lesson being - keep an eye on the oil level and top it up. I was complacent and never bothered checking; if I had, for the sake of a litre of oil from time to time, I'd have saved the cost of fitting a replacement box.