The most notorious example of this is phase dagger vs. greater moloch. Phase dagger adds monster PV to damage... so phase dagger does 1d4 + 100 vs PV 100 to give 1d4 as expected. But if phase dagger is blessed, then the bonus gets multiplied through: (1d4+100)*1.5 = 150 damage vs 100 PV gives 50 damage total.
Weirdly, IIRC, this doesn't apply to regular weapons of penetration. These just ignore PV outright. No idea how it works with monsters.