Caltrops

Catltrops are small, sharp metal spikes that can cause a lot of injury to the unwary adventurer. Essentially, they are four metal spikes welded together in a way that when spread across the ground they lay with one point upright, waiting for unsuspecting feet to walk onto them.

Caltrops are not considered a trap, thus the spell Destroy Trap can't remove them. However, once discovered, it is easy to just remove them manually. However, there is a second sort of caltrop that is made invisible by magic, becoming more difficult to detect. They are frequently laid out in dungeons, where low light makes it difficult to notice them until walked upon.

Trivia

 * Caltrops are the successor to bear traps, which worked similar, but were more difficult to remove.