[Fern]

Defeating an enemy with Nefarious Riff can cause them to be able to take their turn while at 0 HP.
 
Steps to reproduce:
1. Fight a Player Character
2. Quickly use Nefarious Riff to kill them, make sure they're the next character in turn order before they die
3. Observe the person be able to act with 0 HP for one action
 
Can also occasionally happen with mobs. It's probably because it does damage before the animation finishes.