The bug

Very large amounts of block entities (in this case, furnaces) cause intense server-side lag (MC-117075) and use up excessive memory. This will cause the world or certain chunks to fail to save or corrupt, and the game to run out of memory and crash. This does not happen on superflat worlds with furnaces.

To reproduce

1. Create a new world and switch to spectator mode.
2. Execute /fill ~50 ~ ~50 ~-50 ~2 ~-50 furnace and create multiple layers of tens of thousands of furnaces in the world.
3. Continue to execute the commands up to y=255 until about a million furnaces are placed.
4. Notice how extreme lag occurs, and observe the Mem usage steadily going up.
5. Quit the world.
6. The world will get stuck in the saving world screen, until failing to save and crashing due to java.lang.OutOfMemoryError: Java heap space.