Falling thorns
Cactuses are very interesting plants, they can survive in hot, desert climates and are very difficult to kill. In fact, any animal that tries to eat them or damage them in any way first has to go through the thousands of thorns that pop out of the plant. They can definitely hurt a lot if they get you.
That is why only animals who have adapted and evolved over the years know how to skillfully get the nutrients inside a cactus, but other than that no one else should touch them. Could you imagine one falling on top of your head? It would probably turn you into a human porcupine but in all the worst ways.