Chandra
In this instance, it was sheer clumsiness.
I fluffed a double de-clutch from 3rd to 2nd, and there was a nasty "GRRR" noise. After that, I couldn't engage 2nd without more nasty noises: the synchro didn't work at all. Funnily enough, a week or so later, it just sorted itself out, and the synchro worked perfectly again. Spooky.
So I consulted Geoff. His verdict was that I'd snapped a tooth off 2nd gear, and it had got stuck inside the synchro hub (hence the nasty noises when trying to engage). Then the tooth dropped out of the hub, and everything was fine - only with a few less driving surfaces.
Despite the magnets in the gearbox case, I was a bit worried that tooth debris would be wandering around the gearbox case, looking for trouble. So I decided to get it fixed. The clutch needed doing anyway, you see.
The moral of the story is that you should go easy with your gear changes. And if you lose a synchro for a while - but then get it back, you've probably lost a tooth. Even if you don't get it repaired immediately, I'd suggest you drain the gearbox oil, then maybe you can retrieve the big bits.
Ben