In the episode where the gang is on the top of the roof looking at the stars, there is a minor error. The last ones to leave the roof are Rachel, her boyfriend, and Phoebe and when they leave the door is wide open. Ross is on the other side walking toward Joey who, when the 3 people left was near the edge of the roof. Joey bends down and starts playing with a bug stuck in tar (hidden from view) and then comes back up when Ross says that he thinks he sees the comet. Joey uses the pipe to look at the comet.

How did Joey get a hold of the pipe when the door was last seen wide open and Joey never left the view of the camera except when bending down? Did he crawl over to get the pipe? If so, how come it was not seen in the background?