If you fill an area that includes a block on a northwest chunk border and y=80, 96, 112, 128, 144, 160, 178, 192, 208, 224, or 240, with cave air, then fill that same area with any block replacing type=air, it will replace any and all blocks on a northwest chunk border at all of the specified y levels, along with three more blocks which create a square. One corner of this square is the block that is on the northwest chunk border and the opposite corner is the northwest corner of the fill command. This blocks shouldn't have been filled, as they were supposed to be cave air, not regular air. Even more strangely, if you then try to fill that same area with cave are again, the chat will say that the blocks are filled, but they will still be there. They are real blocks too, and didn't disappear after leaving and rejoining the world.

Reproducing the bug:

1. Create new minecraft world in creative, go to 0, 0
2. Use command "/fill 1 0 1 -4 255 -4 minecraft:cave_air"
3. Use command "/fill 1 0 1 -4 255 -4 minecraft:gold_block replace air"
4. Use command (again) "/fill 1 0 1 -4 255 -4 minecraft:cave_air"