Small Pit Trap (4e Trap)
From D&D Wiki
This small side-closing pit trap is quick to create, and one of the basic designs in a trapsmith's arsenal. It is a small hole concealed with leaf litter or debris, which impales the victims foot when stepped on.
It occupies one square, usually dug singly on a forest trail or in a scattered group in clearings. Since they don't do a lot of damage on their own, they are best used in conjunction with an ambush.
If detected, it can be safely triggered to mark its position. Once triggered the square can be entered safely - it's small enough to step around.