The tiger keelback (Rhabdophis tigrinus) has been annoying pedants the world over for years by being both venomous and poisonous. It’s not very big (2-3 feet) and subsists mostly on a diet of amphibians. It’s not terribly aggressive, strongly preferring to either play dead during low ambient temperatures or run away during higher ambient temperatures.
Above: Part of the “no, seriously, I’m dead” display is flattening out their necks to better show off their orange stripes. There’s apparently some question as to whether this is an aposematic display, but given the fact that it’s venomous, poisonous, and how many other snakes that do the neck thing as a “fuck ooooooooooff” display, I’d say the answer is probably that yes, it’s an aposematic display.
They take the playing dead thing pretty seriously, too. I mean, they go limp, which is kind of hilarious in a venomous snake.
Above: What passed for acceptable science in 1983.
There are rattlesnakes out there shaking their heads at this snake.
Part of the lack of significant aggression is probably due to the fact that it’s a rear-fanged snake, which is an arrangement that’s pretty effective if you’re killing small animals to eat them and less desirable if you’re trying to like, keep something thirty times your size from eating you. Rear-fanged snakes tend to have to open their mouths a lot wider to get a decent fang-grip on something, and the venom delivery mechanism can be a sad mockery of efficiency by occasionally requiring the snakes to actually chew on something to get it properly envenomed*.
Above: Western Hognose snake, which is venomous but not in a way that humans need to care about, displaying its sad little fangs.
Front-fanged snakes generally have both an easier time getting a good strike in and a much better injection mechanism.
The poisonous part comes in due to these snakes being in the habit of first eating poisonous toads and then taking the toads’ chemical defenses as their own. Unlike the garter snakes who eat rough-skinned newts and wind up just generally toxic, tiger keelbacks have special glands where they concentrate and store the toads’ poison. Their nuchal glands are found running down either side of their necks, and woe betide you if you break the skin over them. (There aren’t actually convenient ways for the snake to discharge the gland without tissue rupture.) Mothers can and do pass loads of the toxin on to their clutches, assuming they have any to spare, which tides the snakelets over until toad-hatching season brings a ton of snakelet-sized poisonous toads for them to eat.
How confident are these bastards in their nuchal glands saving them?
Above: A snake smacking its neck into something that’s annoying it.
So the answer here is: Extremely confident.
Of course, the functionality of these glands depends on the availability of and their ability to catch the poisonous toads they get their poison from, but the great thing about the toxin is that it’s massively unpleasant (foul-smelling, produces acute burning sensation upon contact with mucous membranes, capable of making you quite sick if you eat it), but it’s probably not going to kill a large animal. So once you’ve had one run-in with a locked and loaded tiger keelback, you really have no particular desire to bite one again, and seeing those stripes come out is going to bring back some really unpleasant memories no matter how much poison that individual snake might be packing.
*Coral snakes do this, for instance. If you feel the need to let a coral snake bite you, please do not sit there and let it chew on you just because it is tiny and kind of ridiculous.
[Snake-Humiliation Olympics photo from “Death-Feigning Behavior of the Japanese Colubrid Snake Rhabdophis tigrinus.” Akio Mutoh. Herpetologica, 39:1 (Mar., 1983), pp. 78-80; Neck-butting photo from “Nuchal glands: a novel defensive system in snakes.” Akira Mori. Chemoecology, 22 (2012), pp. 187–198.]