With Maha, Its Feathers Night and Kaervek, the Spiteful on the board, as I understand it, opponent creatures should die immediately when they enter the battlefield. Instead it appears Kaervek applies -1/-1 before Maha's 1 toughness applies. So for example a 3/3 will enter the board and still lives as a 2/1 instead of coming in as 3/1 and dying to Kaervek's -1/-1.