when you usually make sheep follow you, they will be able to jump up a block, but with snapshot 14w31 minecraft, when sheep follow you and come to a block, they will try to jump on it over and over but simply not be able to.

here are the steps to reproduce this:
1. get some wheat
2. find a sheep and make it follow you
3. come to a 1 block high "wall" and the sheep will not be able to jump over it