Misfire Blowgun (3.5e Equipment)
From D&D Wiki
Misfire Blowgun: The Misfire Blowgun was a trap laid by a cruel shaman. His people were being exterminated and rumor among his people was that the invaders were taking trophies. The shaman created many of these blowgun that appeared to be functional upon use and he decorated them so that they would catch the invaders eye. However these blowguns were more then mere trinkets they were in fact powerful weapons.
The first three shots fired by a new holder are unremarkable, the weapon itself is a +2 enhancement. However the fourth shot and ever shot there after backfires, it's considered an automatic hit dealing damage as normal damage to the user.