It happens in Group Instances where the Boss turns from NPC/Ally (Green) to Hostile (Red).
If you throw Lethal Concoction on the floor before the Boss turns Hostile (Red), Lethal Concoction is not applied and the boss does not take damage from the LC pool.
This bug is found in the following Group Instances:
Othallo Guardhouse
01. Gaoler Brin
02. Warden Teathon
The Vault
03. Deformed Plagued
04. Captain Johnston
Garai Colosseum
05. Undead Troll
06. Giant Scorpion
07. Lord Horus
Secluded Valley
08. Duoberus
09. Dark Wraith
Temple of the Three
10. Terra
11. Zephyr
12. Viridian
Also happens with Bosses in the Eldevin Arena
13. Terra the Earth God