Yawnicus maximus I know, but I have a mudguard bolt where the head just snapped off in my commuter fork leaving a stub.
Molgrips – not shifting.
I filed some flats into it and put a 4mm spanner on it – rounded straight off.
Then I hacksawed a slot into it (quite beautifully central, I was proud), got a screwdriver onto it – one side just sheared off.
Then I drilled a 3mm hole through what was left, hoping the heat would crack the corrosion and loosen it, no joy, the rest of the stub just ripped off.
Any bright ideas before I drill the whole thing out and try to find a longer bolt & low profile nut??? I do not have access to a helicoil.