Be very careful what is said about Fenestron Stall, to newbie's.
There is a phenomenon where the "acceptance angle" at the shroud permits the tail rotor to accelerate significantly to such an extent that the aircraft will rotate well in excess of its operating limit. (The cab can rotate around its axis atleast once or even twice IN SECONDS before the uninitiated pilot knows what is happening).
As (quite rightly) mentioned, this is NOT a design flaw, or LTE. It is, how shall we say: "a shrouded tail rotor aerodynamic phenomenon" which ALL shrouded tail rotor pilots MUST be aware of.
Really? And what's happening to the main rotor RPM at this time?