The strangest thing happened a few days ago. I was removing miscellaneous weeds from a bed of day lilies when I ran across several thorny weeds like the one pictured here. I wadded up a paper towel to protect myself from the thorns (which are located along the length of the stalk) and went to work removing them. One of them, though, wrapped slightly around the paper towel, jabbing a thorn into the large middle joint of the ring finger on my left hand (well, it would be the ring finger if I wore a ring, I guess).
Okay, I got stuck by a thorn--no problem. But it turned out that there was a problem. Within three hours, the finger showed every symptom of being severely jammed (although it wasn't): the joint itself became stiff, reddish-purple, and painful to move, while flexibility in the joint was severely limited. (Susan commented that the symptoms were also remarkably similar to osteoarthritis.)
I've never had this problem before. I've been stuck by this plant's thorns in the past with no odd side-effects; in fact, I was stuck on the back of the hand by the same plant on the same day, and no reaction at all.
Took about 24 hours for the hand to return to normal. When it did, it was rather sudden; almost 23 hours later, all the symptoms were still there. An hour after that, they were pretty much gone.
Have no idea what caused the odd reaction to a routine thorn scratch... Probably never will. I've considered sticking another finger with a thorn from the same plant to see if I can duplicate the reaction, but it was unpleasant enough that common sense tells me to reconsider...