Decided I would use unrendered world holes while flying in Creative to find abandoned mine shafts to explore, see what kind of things I could find. Dug straight down under oceans to shafts. Found I couldn't move through some shafts. As I tried to place torches or click ahead of me, invisible blocks would appear when the torch placed (some wood, some stone, etc) or be destroyed (when left clicked). Placed a torch later in a tunnel and the stone I placed it on became coal ore. Did the same for the block beside it and became coal ore too. Clicked around and some changed others didn't, as if a coal seam was invisible, till I placed a torch on it. Replicated easily with little tunnel exploration. Did not test in survival.