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usually machining too much material from a cast iron flywheel.

I'd say rapid heating and cooling, particularly on a lightened flywheel. They tend to shatter - just too much mechanical stress on them. Like doing a 10min burnout continually slipping the clutch on and off....(I saw a VL Turbo spit a flywheel doing exactly that a few years ago) - Funny yet potentially deadly.

In the case with Ian's B310, it was a shoddy job done on the fly wheel bolts. Someone and it wasnt me, put non high tensile where high tensile bolts should have gone, plus they had welded the bolts together.

Well look at that, almost looks like a clean break. That's scary