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Built a manamune and had this happen every single game I used it in. It only gives 1% of your max mana as damage, but all the values on the item itself show that it is giving you the normal 2%. It's either that, or for some reason it isn't giving the 20 damage you get just for building the item. Either way, I'm always roughly 20 damage short of what the values add up to be on this item.