Caltrops

Catltrops are small, nasty things that can cause a lot of injury to the unweary one. Essentially, they are four metal spikes welded together in a way that when it lands on the ground, on spike always points upwards, waiting for unsuspecting feet to walk into them and cause serious injury.

Caltrops ar enot considered a trap, thus the spell Destroy Trap can't remove them. However, once discovered, it is easy to just remove them manually. Note however that there is a second sort of caltrop that is made invisible by magic, becoming much worse than the regular kind, especially since they are often laid out in dungeons, where low light makes it difficult to notice them until walking into.

Trivia

 * Caltrops are the successor to Bear Traps, which worked similar, but were more of a pain to remove.