In a new Hardcore mode game generated in 1.8, a spherical cloud moved in front of/around me on my second time playing in it. In the inventory screen I am invisible. The cloud comes through walls, but I didn't get a screenshot of it. I deleted the world and made a new one in creative using the same seed as before. I didn't move. Even looking straight without moving, the terrain was changing frequently but not rapidly between different biomes and transparencies. The wire block frame highlighted invisible blocks, but you could see straight through them. I took four screenies looking in the same general direction without moving anywhere. I never picked up any items and was not holding anything when these were taken.