Whenever a Zombie Pigman climbs out of lava on to a block the same height as the lava, it will get hurt from fall damage. They will continue to do this until they die or they de-spawn. I'm not sure if this is fixed in for the 0.12.1 release, but if not then it will be a problem in the nether because zombie Pigman will keep dying. I attached a photo of the place in which they were climbing out of lava onto the nether brick. I'm not sure whether this will affect Steve or any other mobs once swimming in lava is added.