Steps to reproduce:
1. Tame a horse.
2. Place some farmland.
3. Jump down on that farmland so that the horse entirely lands on it and tramples it.

Expected behaviour:
Either the player dismounts the horse or nothing happens.

Actual behaviour:
The dismount animation plays and the player can move freely, but the world behaves as though the player is still on the horse (e.g. can't break blocks and attack mobs outside of a certain range, mobs don't react to player movement, etc.) and the horse stays in place even if blocks below it are broken. Sneaking fixes the issue.