The bug

When an entity has its head inside farmland and the farmland turns into dirt, the entity will be instantly (in less than 1 game tick) teleported above the dirt.
The teleportation works also with entities with NBT NoAI:1. They get teleported up too.

How to reproduce

[e.g. Floor is at y=10]
1. Place a farmland 1 block above the floor. (e.g. x=10, y=12, z=20) [2017-08-28_11.41.47.png]
2. Summon an armor stand on the floor with the head inside the farmland (e.g. x=10, y=11, z=20) [2017-08-28_11.42.23.png]
3. Jump/place a block above the farmland to turn it into dirt. [2017-08-28_11.42.36.png]
4. The armorstand instantly teleported up.

Testing results

• We can teleport an entity with farmland infinite times a tick, but due to the fact that entities can't go through more than a subchunk every gametick, the maximum (upwards) speed is 320m/s.
• Farmland can be transformed by using pistons to move blocks above it, or by moving it below a block, and the bug still works.
• It is possible to build a survival vanilla friendly 320m/s elevator (a subchunk per gametick), moving farmland below signs in the right order.
• Items can be teleported by placing farmland above other farmand using the right update order (this also works at a subchunk per gametick).
• Endermans (3 block high mob) gets teleported using a farmland every third block.

Why it happens (my thoughts)

I think it's a bugfix ended up badly: they tried to fix MC-104259 and they failed.

By the way, it only works with farmland and not with path blocks.
I made a video about to show the bug and to show the testing, HERE (turn on english subtitles).