In previous versions, while riding horses or donkeys under trees of 2 block tall gaps, the player's head would clip through the blocks. But in 19w14a, it pushes you down into a swimming animation, also putting the first person perspective inside the horse/donkey. This makes it harder to traverse forested areas as you can't see where you're going.

To recreate, get on a tamed horse or donkey, and ride underneath a gap of two blocks tall.