Cheshire Cat’s poisoning is undispellable and lasts for 4 turns. This also means, that any additional specials with the same effect, e.g. poisoning, have no effect, and Cat’s one stays.
This leads to an unpleasant situation, when, for instance, Sartana is shooting on the 4th (the last) turn of Cat’s special, Sartana’s poisoning will not be applied at all, and poisoning ends on the next turn instead of lasting for next 6 turns. Same goes with Proteus, Layla and other similar heroes.
I would expect that the poisoning continues, but becomes dispellable.