This was a complex cleanup shot, done at CG Lab in early 2011. We had to remove:
Rigs.
Tracking markers.
Wires.
Plastic water bottles and other elements left on set.
Why one shot takes that long. In a long, busy shot, every item to remove has to be tracked and painted out across hundreds of frames. The patch has to match what sits behind it, frame after frame. One miss shows up the moment you watch it at full speed.