So here's a thread about what plants or animals you'd least want to accidentally (or I guess if you're daring/stupid enough; voluntarily) come in contact with. I was prompted to start this thread after watching a documentary on Box jellyfish. I sure as fuck feel bad for those who live in Australia. They have some bad ass shit that live there.
So to kick off the thread here are two things I hope I never have to deal with:
Box Jellyfish (Chironex Fleckeri)
Chironex fleckeri - Wikipedia, the free encyclopedia 
This thing is so toxic that a single tentacle has enough juice to kill 60 adult humans in as little as 3 minutes!
And as for the plant, we have the castor bean plant. It apparently contains a poison called ricin, which incidentally was used to kill a Bulgarian writer in the late 70s. Fucked up thing is apparently this thing is planted in parks in most cities. Though death from castor beans are rare, I sure as shit wouldn't want to risk it.
