It has been reported to proc even on "Grey recipes", indicating that the difficulty level, and perhaps the item level, of the recipe has no influence on whether or not it occurs. Patch 3.1.0 doubled the amount of flasks produced by all recipes for flasks in the Wrath of the Lich King content. (An x2 proc is now x4, an x3 is now x6, and so forth.) Cataclysm and The Burning Crusade flasks remained unchanged.