From 3.5 -
A character has a 5% chance of exposing himself to a poison whenever he applies it to a weapon or otherwise readies it for use. Additionally, a character who rolls a natural 1 on an attack roll with a poisoned weapon must make a DC 15 Reflex save or accidentally poison himself with the weapon
5.0 House Rule?
I like this "a character who rolls a natural 1 on an attack roll with a poisoned weapon must make a DC 15 Reflex save or accidentally poison himself with the weapon"
but I don't like the 5% - does not count for the Proficiency
1 in 20 chance for applying poison to accidentally poison self
A DC of 10 is 50% chance of wasting the poison and possible accidentally poisoning self - reduce it by High Dex - Proficiency Bonus +2 to +5 and Expertise; reducing the accidentally poisoning self to zero
A Failed Poison Kit Check to apply the poison means a waste of the poison
If the character fails the check by 5 or more, the character is subjected to the poison
This would make High Level Users of Poisons immune to the "accidentally poison self", say from using poison over many years the Assassin has finally stopped nicking him or her self
Remember you can not Fumble a Skill Check but you can Fumble an Attack Roll