Okay but platypus venom is actually one of the weirdest things about them.
So like, everyone knows how famously weird platypuses are. Semi-aquatic egg-laying mammals with duck bills and beaver tails, but too few people mention how strange their venom is.
First of all it's only found on males. I'm no scientist but from my (limited) research, I can't find any other example of a vertebrate where only one sex has venom. Venom in mammals is rare enough already anyway, but sexual dimorphism in the venom in a mammal is all but unheard of.
Next, platypi primarily use their venom on each other. The males produce venom from their leg knives year round, but venom production goes in to overdrive during mating season, leading scientists to believe that they mainly use it on each other to compete for mates. As an additional not here, not only is it injected through their legs, but they use their legs as a vice, trapping their victims between them and repeatedly stabbing.
Now, what the venom actually *does*. It will temporarily paralyze other platypa, but will quickly kill other small animals. There have been no recorded deaths on humans from platy-venom, but if you get stung by one, you'll wish you had been thr first. The pain is "immediate, sustained, and devastating". Fortunately, there have been so few examples of humans being stung that the results are not very well-researched, but what has been seen is enough. It will cause the afflicted area to swell up with extreme pain for days, weeks, or even months. Without end. Even 25 years later there have been reports of continued stiffness and pain in afflicted areas.
I've saved the worst for last though.
It's immune to painkillers.
That's right, even morphine is inadequate to stop the pain from these guys. You get stung? Debilitating, inescapable pain for months on end with lifelong aftereffects.
Tl;dr: you're right to be wary of their venomous knees. Never go near a platypus unless you know for certain it's female.
(Edit: clarified the bit about sexual dimorphism in vertebrates)