This blooper happens at the beginning of Talitha Cumi I think it is (either that or Herronvolk in season 4).

You remember the phone repair guy at the beginning of the episode, well, when he is surrounded by the clones if you look at the floor you will see every-bodies shadows - but not that of the pole behind them!

The post-production crew forgot to add the shadow back on after adding in the clones, although they added their shadows in seamlessly.