We're using the GameMastery Critical Fumble Deck.
The Disrupt Undead spell is, according to the description, a ray of positive energy. If it hits an undead, it does 1d6 damage.
No mention is made of any effect if a non-undead is struck, as can easily happen in a fumble.
To avoid delaying the resolution of combat, we simply had it do nothing. But as positive energy, should it have some effect on non-undead or objects ?
Channel Energy has set the precedent that summoned positive/negative energy isn't "double edged" anymore.
So the ruling it did nothing to non-undead seems to be the correct one.