Mobs go though blocks

Is this supposed to happen?

I expected the horses not to go though walls. (blocks)

When playing the new snapshot 13w21a I spawned horses with spawn eggs. After a while they were wondering about near my building (made with glass and iron blocks) Then they started to go though the wall (blocks) taking also damage.

How to trigger glitch
1. Spawn a few horses near a wall
2. wait till the horses go though the wall. (blocks)