In the previous episode, S5E14 "The One Where Everyone Finds Out", when Ross goes to visit ugly naked guy's apartment, the apartment number on the door was 201. In this episode, Ross moves into the apartment and now the apartment number changes to 3B.
Ross' apartment is in the middle of the hallway. When Ross goes across the hall to check out the party for Howard the handyman, the camera cuts back to Ross several times with the apartment doorway behind him. The first time you can see all the way back through the open door and into Ross' apartment, but every time after that, a corner of a wall has been placed in the doorway that would make it look like this apartment is located at the end of the hallway.