If the player tries to hop on to a Block that ist not full hight (15/16 pixel) he gets pushed off.

This occurs on farmsoil and village pathways.

If the block is not level with the surrondings, it won't let the player get on.

If the block is level with the ground, but has trapdoors on the side, it "sticks" the player to the side with the trapdoor.

The only way to unstick oneself is to place a block next to the 15/16-block.