When a entity, that has a different posture while riding a mount, is in a mount where the entity's seating location is quite close to the mount's bottom, the entity's hitbox-bottom is placed lower than the mount's hitbox-bottom. This may create situations, egde-cases, where the entity's hitbox is placed within a block that will damage it.
The blocks I have tested to work, are trapdoors, and a daylight sensor, so that is blocks that are less than a half block tall. (See image with player in a boat, on a half-slab)

Steps to reproduce:
1) Get a boat, or a minecart, or another mount that may work.
2) Sit desired entity in the mount.
3) Move the mount to a block, shorter than a half-block tall.
4) Observe the entity's hitbox being lower than the block, that the mount resides on. (Using F3+B)

This has neither been tested for any versions before 1.21.1, nor any newer snapshots.