Insufficient backlash

[CheeserBeezer]

This can be caused by old age and fair wear and tear but can also be caused by insufficient backlash between the magneto gear and camshaft gear. Always check for a bit of backlash between the gears in all positions. You can also run a piece of paper through the gears and make sure that it's always loose. There is an engineering term for pressure on a shaft which causes cracking and eventual failure but I've forgotten it!

[BagONails]

Stress reversals leading to fatigue failure I would think if the gears go tight then loose then tight again on every revolution?

[JulianS]

This one broke at the other end.

[groily]

Is that a 15mm dr end CB? Looks like  . .
The ones I have seen have all been 15s, and had been running with ATDs. (Not that that's cause and effect in and of itself of course - just something that struck me.)
Good luck with mending it if you're going to - not the best fun job.

Not seen a contact breaker end shaft break though, have to say. Ouch!