2 mobs trying to go through 1 block wide path

It's like slapstick: When two mobs try to go through a one block wide path, they block each other and just stand still. Doesn't matter if it's a zombie trying to eat your brain or a cow following the wheat in your hand.
Video (German, but doesn't matter): youtu.be/PxbspSNyQVU?t=5m16s