I once have lost the left part of the tail plane (with its vertical stabilizer) of an AS 365 N2.
I was performing an air test including a vne. I initated acceleration to Vne from 5000' with a 25 / 30 deg. dive.
I didn't notice anything very unusual except a slight unstability in pitch with a marked tendency to pitch down at about 3000' and a much longer recovery trajectory, levelling at about 800'.
Marks on the tail fin indicated a short miss with the tail rotor.
The cause of this structural failure have been found to be a weakness in the root section of the tail plane (made of carbon fiber) maybe due to a former shock on the tail plane.