A lot worse than a rock in your shoe
Vietnam, Marines of Company H, 2nd Battalion, 4th Regiment, walk through a punji-staked gully; 28 January 1966. Note the M14 battle rifle, Marlboro (they were issued in packs of 5 in C-rats) and bare M1 helmet.
Punji sticks are ancient anti-personnel devices, with the British reportedly encountering them in Burma as far back as the 19th Century and, as noted in our post on the frogmen of Balikpapan, the Japanese used them extensively in WWII. Today they are banned from use in warfare under Protocol II of the UN’s 1980 Convention on Certain Conventional Weapons.
Of course, those who are most likely to use them never had much use for what Geneva had to say, anyway.