I spawned a bat in a 2x2 hole to the void and covered it with glass. The bat flew down the hole so I made a 1x1 hole to the void and jumped down it. When the bat came down, it went a few blocks away and didn't just hang on the bottom of the world, it hung a block under the world. I took a screenshot and force-crashed Minecraft just to get all of the details.
MC-47988
Bats Can Hang On An Invisible "Block" Below Minecraft
Resolved
Invalid
0
The bat is hanging at negative coordinates:
The last block is placed at y=0, so there is no valid position below that.
In the 1.8 snapshots entites are deleted / killed if they go to negative coordinates.