I walked into a Stihl dealer's repair shop last year at their invitation so that I could meet the senior mechanic. A standout feature I saw lying all over the place was broken chain brake handles. They were everywhere.
one last comment on the broken Brake Handle. Upon examining the fracture surface, it appears the plastic is partially melted. The proximity to the exhaust makes me think that the heat from the exhaust may have partially melted the handle, and combined with the stresses induced by operation, the plastic failed.
Hmm... The muffler's exhaust port is far forward from that position on the chain brake. Unless there is a hole in the muffler near the same area, I doubt there would ever be enough heat to cause this.