Steps to reproduce:

1. Build a 6x6x6 room with opaque block walls.
2. Add a small entrance to the room (door, fence gate)
3. Spawn an Iron Golem into the room.
4. Aggravate him (punch) and quickly leave the room.
5. Stand next to one of the exterior walls of the room.
6. The Iron Golem will hit you through the wall.