Plot hole: The murder weapon during Miss Fisher's demonstration hardly appears to generate enough momentum not just to send a projectile across the room with lethal force, but also do it achieving full penetration through clothes, disappearing entirely inside the body so the coroner wouldn't notice. That's assuming that the coroner did not even bother to in fact open up the cadaver and perform autopsy, which would be strange, especially with a weapon of undisclosed characteristics, when finding out more about the depth and damage to internal organs would have been crucial. And with any autopsy the dart would have been found, solving the case almost immediately. (00:49:45)
Plot hole: Talking about the murder weapon, the coroner says that the wound was a "horizontal" stab. Nerine's confession is ruled out because she mimics the attack at an angle. Funnily enough though, during interrogation, Jack himself makes a remark about the stage being a spot overseeing the room, and the killer was in fact on stage, and standing. When we see briefly the flashback of the murder, it's clear that the trumpet is way above the head of everyone dancing. So the projectile wound would hardly be "horizontal."
Answer: Louisa's watch was broken during the struggle to get away from her attacker.
Bishop73