I've been looking through the Magic Weapons for Pathfinder, and came across the Furyborn enchantment. Basically, whenever you damage someone, you get a +1 Enhancement bonus to your weapon against that single target (to a maximum of +5). However, a magic weapon already has an enhancement bonus of up to +5, so is this a rare instance of a stack-able bonus, or is this just meant to be a lower-level supplement enchantment until the party can afford better weapons?
Furyborn improves the existing bonus.
From the Furyborn enchantment description (emphasis mine):
This special ability can only be placed on melee weapons. A furyborn weapon draws power from the anger and frustration the wielder feels when battling foes that refuse to die. Each time the wielder damages an opponent with the weapon, its enhancement bonus increases by +1 when making attacks against that opponent (to a maximum total enhancement bonus of +5). This extra enhancement bonus goes away if the opponent dies, the wielder uses the weapon to attack a different creature, or 1 hour passes.
Note the wording here. It's not giving you a new enhancement bonus, it is increasing the weapon's existing enhancement bonus (all magic weapons must have a plain enhancement bonus of at least +1 in order to add special abilities). It can only increase the weapon's enhancement bonus up to +5, the usual cap for enhancement bonuses from magic weapons.
The purpose of this weapon is to essentially grow from the weapon's regular enhancement bonus up to the maximum +5 against more difficult targets (i.e. targets that take more than one hit to defeat). If your magic weapon already has +5 enhancement bonus then the furyborn enchantment is completely useless.