When your work involves balancing on thin threads, wrapping up struggling prey, and trying to bite it, you can expect a few accidents -- especially if the prey can bite back. One group of spiders has an effective solution for when they get bitten by venomous prey.
Orb weavers are a group of relatively large and colourful spiders. They have some good features, such as a tendency to set up shop in gardens and eat pest insects. But they also have some bad features, such as a tendency to create nearly invisible webs that any innocent person can blunder into. At least they don't beat up on the weaklings. They regularly eat prey twice their size and will go after prey that's venomous.
There are hazards to this. Sometimes the spiders get bitten by venomous prey, or other venomous insects. The bite usually lands on one of the spider's legs. In response, the spider jettisons the leg. Lizards are most famous for autotomy, the dropping of a limb as a defence mechanism, but they usually lose their tails in response to a mechanical stimulus. Spiders seem to do it as a way to jettison the venom before it can get pumped into the rest of their body.
Some spiders don't manage to drop the leg in time and die off, so it's not a mechanism of the poison. What causes a spider to detach a limb? A group of scientists injected spider limbs with various poisons and found that they drop legs in response to multiple types of venoms, including bee and wasp venom. The spiders also jettison the limb when they are injected with components of the venom, including "serotonin, histamine, phospholipase A2, [and] melittin."
These aren't necessarily fatal but they are the components of venom known to cause pain to humans. So not only can spiders detach their own legs, they might feel "pain" the same way that we do. Our suffering and a spider's may be the same.