When Cassiopeia has poisoned and enemy and uses twin fang on them it wont always give it the fast resets, and will use its normal cooldowns. It doesnt seem to matter if it is on a minion or champion or monster.
Play Cassiopeia in blind pick 5v5
Use either your Q or W on any opponent
Use E on the poisoned target.
It usually doesnt give you any reset and you get murdered.