When jumping on tilled, irrigated farmland, with crops, players and mobs glitch downward.

To reproduce:

Create a structure.

As roof, place farmland.

Irrigate the farmland.

Under the farmland, there must be air.

Plant a crop.

Jump onto the crop.

Expected result: Player lands on the farmland. Farmland is turned to dirt. 

Observed result: Player glitches down through the farmland. Farmland turns to dirt.

Works for sheep too.