Prickly-pear Cactus (Opuntia humifusa)
This hardy cactus is widespread in eastern North American and is especially common in scrub. Florida scrub is an inhospitable habitat to walk through, and prickly-pear is one of the reasons. The pads, armed with barbed spines, seem capable of leaping a foot or more to imbed themselves in human flesh. Prickly-pear will often attach a pad surreptitiously to the back of a person's boot heel, then when the person crouches down to see something on the ground, the pad imbeds its spines into the soft flesh of the person's bottom side. There's more than evil to this behavior, though. As anyone who's propagated cactus knows, the pads are capable of rooting where they fall and we and other animals help spread prickly-pear whenever we detach a bothersome pad and throw it on the ground. Perhaps we can take some satisfaction in knowing that the pads and the fruit of prickly-pear are edible, and actually quite tasty.
See Floridata's profile of Prickly-pear Cactus (Opuntia humifusa)