Realms Survival Server, duplicated in single-player creative
When a player, villager, or (likely) any mob in a minecart is on rails on a 1-thick dirt/farmland layer above a layer of cart rails immediately below it (as in a wheat autofarm), they may fall through farmland when pushed off a regular dirt block onto the lower-height tilled farmland block. Players wearing Elytra walking/jumping off dirt blocks onto tilled farmland may also fall through and go into elytra-flight mode (body horizontal rather than vertical), but I only saw this once and could not duplicate it myself.
What I expected: Villager in mob cart being moved into a farm can be pushed off the end of a rail line into the farm area and onto tilled farmland without falling through.
What I got: When rail partially under the villager's cart was broken, if the villager in the cart was pushed off the edge of the dirt block the rail had been on, he falls through the tilled farmland blocks onto the layer of cart rails below and starts riding around with his head inside the farmland blocks. He does not suffocate there, but if the rails lead to areas with solid blocks above the rails, he can take damage and die.
To duplicate glitch, build a layer of densely-laid minecart rails (on every block in the given area) with a layer of dirt blocks above and a water source to hydrate the farmland. Till most of the dirt blocks into farmland blocks. Place rails on one or more of the remaining dirt blocks, and place a villager, player, or other mob into the cart. Push them so they are half off/half on the rail at the edge of the dirt blocks, hanging over a tilled farmland block. Break the rail. Now push the cart fully onto the farmland block, cart will glitch through the farmland block and begin riding the rails on the layer below. Tested with both villagers and a player (me) in the cart.
While helping me duplicate this glitch, another player who happened to be wearing Elytra at the time fell through the farmland blocks in a similar manner to the villager in a minecart. She was walking or jumping along a border between dirt and tilled-farmland blocks at the time. She ended up horizontal as if flying, but inside the farmland/dirt block layer. She was unable to "land" until the blocks she was inside were broken by another player. We were unable to immediately duplicate that part of the bug, however.
See attached screenshots of my efforts to quickly duplicate the bug in a single-player creative world, with a villager in a minecart trapped with his head inside the dirt blocks after falling through.