Visible crew/equipment: Right after Rachel says, "Do you know that feeling when you're trying to blow a Saint Bernard out your ass?" the camera cuts to a view of Monica and Chandler. Behind Monica and Chandler, part of the set is visible, along with a man on crutches.
Visible crew/equipment: Right after the "I'll Be There For You" intro, the camera shows the view from outside the "Central Perk" café. As it does so, a couple of stage lights are visible through the upper part of the window, obviously part of the set on the inside of the building.
Visible crew/equipment: When Rachel exits her bedroom and says she's going to London, you can see tape markings on the floor by her bedroom door and the end of the set. Furthermore, you can see a chair which is likely part of the Central Perk set. [Widescreen version].
Visible crew/equipment: In the scene where Rachel is trying to hide Ross from Paul, when he's about to go into Paul's bedroom after he says "Did you really hear a bear?", look at the window behind Rachel when the shot is on her. You can see a large light reflected in the glass and it moves slightly (not just in conjunction with the camera, it moves before the camera shifts).
Visible crew/equipment: After Max tells Phoebe that he and David got the grant and are leaving for Minsk on January 1st, when Phoebe goes to see David, just as she knocks on the door a boom mic can be seen moving at the top of the screen, before it cuts to David's closeup.
Visible crew/equipment: In the scene where Chandler pretends that he just got home from Tulsa and meets Joey who is waiting in the hallway outside the apartment, the camera cuts between Chandler and Joey. Every time the camera shows Chandler, there is a wooden wall at the left edge of the screen with an orange tape. This is evidently part of the set, because when the camera cuts to Joey, the wooden wall with the orange tape is nowhere to be seen.