Joanna: What are you doing?
Chandler: I'm getting dressed.
Chandler: Because when I go outside naked, people throw garbage at me.
During the scene when Phoebe breaks into the other Phoebe's house and gets caught, the older Phoebe was telling the younger one that the man in the photo is not Frank Buffay, but is Chuck Mangioni. Right after that, as Phoebe 2 walks towards the camera, you can see that the door behind her moves open, and there is a visible shadow of someone who walks by that doorway.
Matthew Perry's wit is so legendary that the scriptwriters have often incorporated his gags into the show.