Mundo E has no HP cost

Dr. Mundo's E has no HP cost. The tooltip for the ability says #HP for the cost(25 HP rank 1). When ability is activated, regardless of ranks/points into it, the cost is never subtracted from your HP. You'll always get the full effects of the ability, but free of any cost.

From the things I've experienced and tried so far, there is no set circumstance to get it to occur. It just happens in every game, regardless of masteries, summoner spells, or items.