On a similar note, another thorn in my side are stickers.
No, not the adhesive picture whatevers.

These little starfishs are like legos, almost. After a second of walking through barefoot it's like being stung by bees repeatedly.
Run through them barefoot and you bleed almost instantly.
And, guess what, they live up to your name so even with shoes and socks they'll still forget with you.
Oh yeah, and they come in field-sized groups.