Okay, so this happened several times on two independent 1.6.2 servers. When logging out and back in (how long after doesn't matter) mobs seem to move one block to side. It's easily noticeable at cave spider spawner farms. Bug doesn't occur when leaving area (and forcing mob despawn) but when i trapped wither in The End bedrock portal, left dimension and came back he also moved one block side a suffocated. This wasn't happing in 1.5 update (can't tell if it was a problem in 1.6.1).