Mobs phase out of holding areas, then snap back inside them

Multiple mobs in an area will appear to move outside any nonsolid containment area (glass, fences, etc.) and then snap back to a normal position. Effect can be retarded by containing the mobs in solid blocks.

To reproduce:
Build a small fenced-in area in Creative.
Spawn or herd in multiple mobs (tested at 6 mobs and 40 mobs)