Not dangerous, but REALLY painful, but I flung myself into a larger 7x10' patch of stinging nettles in my bathingsuit... It hurt REALLY badly.

I used to purposely swim in the rip current to see how far it would really take me out, swim back in, and then do it again over and over.

I went to Death Valley... And didn't wear sunscreen. All I got was a tiny inkling of color, and despite that, my skin felt like it had been sunburned beyond all reason.