When Phoebe is talking to the Monica in Central Perk, the stone on her necklace is slightly off center, but when Phoebe is talking to the extra that replaces Monica, it is more central.
As Ross and Rachel are talking while sitting on the steps, Rachel's brown paper bag changes positions several times between shots.
When Phoebe and Monica are sat on the couch in Central Perk, Phoebe remarks about Monica's burnt hair. Monica says "I paid to have this done", and when the camera changes and Phoebe replies "Love it", it is quite clearly not Monica sat next to her appearing in the shot. The 'extra' even has her hair styled differently to Monica.
When Phoebe and Monica are in Central Perk talking about firing Monica's new sous chef, a stand-in double for Monica is visible.
When Phoebe and Monica were talking on the couch in Central Perk when Phoebe is trying to convince Monica not to fire her sous-chef, she brings up Monica's Hair after Monica says that she paid to have it like that the camera goes back to Phoebe you see the corner of Monica's face but it's not Monica it's someone else altogether.
In the scene where Phoebe rushes into the kitchen to break up with Tim you can briefly see a hand stopping the door.
When Phoebe is trying to break up with Tim in the coffee house, at the end of the scene at the far left on the screen, you can see the set of Monica's work kitchen instead of the coffee house.
After Ross yells "yes it is too slutty!" and Joey shuts the door, lighting equipment is fully visible in the upper-left hand corner for approximately 5 seconds.
As with all the work-centered story lines concerning Joey playing Dr. Drake Ramoray on "Days Of Our Lives", that soap opera films in California, yet Joey and his friends are living in NYC.
Even if a brain transplant were possible, Dr Drake Ramoray would not be able to continue his career as a neurosurgeon. Because Jessica Lockhart wasn't one, and he has her brain.
After Ross tells Chandler he wants to go to the Hard Rock Cafe again, as they are walking out to the lift in the next scene a boom mic is visible to the top left, above Ross's head.
Phoebe is talking to Monica at Central Perk about the chef that works for Monica, and Phoebe is dating, and Phoebe comments on Monica's hair. Monica tells Phoebe that she paid for her hair to look that way, and the camera cuts to a shot of Phoebe with the side of Monica's face in the foreground ... only it's not Monica - Courteney Cox, it is instead some younger, middle eastern looking woman on the couch next to Phoebe. Cut to next camera angle and it is again Courteney Cox on the couch.
Phoebe mistakes Monica's sous-chef as being her boss even though we later learn in Season 10 "The One Where Joey Speaks French" that she can speak fluent French.
Ross is shocked to learn that Chandler's middle name is Muriel. However, in an earlier episode when Monica and Rachel are competing against Chandler and Joey for the apartment, that's one of the questions Ross made up for the game.