It usually starts when on a hill/mountain, but it has happened in all biomes.  (Has happened on or off a horse) It won't let me auto jump up one block like normal. I have to hit the jump button. Then it seems like I cannot walk or run into a chunk? Sometimes I can go down a ways and it will let me go in that direction once I am past it but then you can't enter that chunk from a different direction. All animals in the chunk freeze up and if I get off my horse and the horse tries to walk into it it will get stuck and start shaking.  If I try to sign out and then get into the world again it will drop me from the sky over the world spawn point then respawn me in a bed. It does this when walking and when riding a horse.  The horse will stay at the place where you were when you left the game and you have to go find it again.