If we're talking about an M20 I've had after market bolts snap off...The plated variety do not seem to have been heat treated after plating...This process should be carried out as the bolts can take on additional carbon during plating, making them more brittle...
Alternatively, over tightening can ultimately lead to failure...In either case the most likely place for them to snap is where the thread ends which is a weak spot...
The correct bolts can be used multiple times if tightened to the correct torque setting (28-30ft. lbs) and not over tightened..
