1. The one with the vanishing necklace - season 2, episode 7

Rachel finds out that Ross and Julie are getting a cat. At first when Rachel says "Together?" she is not wearing a necklace. When the camera cuts back to her saying "Both of you?" she is suddenly wearing a necklace. It then disappears again when she says "Isn't that just lovely?"

2. The one with Monica's stand-in - season 8, episode 5

Phoebe is talking to Monica in the coffee house, who is drinking a cup of coffee. The camera is on Phoebe, and on the right-hand side of the screen is a side view of Monica. However, it is not Courteney Cox that we see, but an entirely different actor altogether.

3. The one with Joey's quick change - season 5, episode 17

Joey is wearing a black top with a zip as he counts the floors of Ross's building to work out where the attractive girl lives. When he enters Ross's building and knocks on what he believes to be the girl's front door, he is wearing a dark purple buttoned shirt. As soon as Ross answers the door however, Joey is wearing the black zip-up top again. Got cold on the way up perhaps?

4. The one with the mystery cans - season 9, episode 15

Joey goes to Monica and Chandler's apartment to get something to drink for the audition. When he closes the fridge he has two bottles in his left hand. When the camera cuts back to him later in the scene, he has two cans in his left hand and is holding them differently.

5.  The one with the self-wrapping present - season 8, episode 2

Monica is sitting in the living room with her wedding presents. She unwraps a large parcel to reveal a brown cardboard box. The camera moves to Joey, and when it moves back to Monica, the parcel is wrapped up again. If only our Christmas pressies could get wrapped so quick.

6. The one with the clones - season 10, episode 20

Some extras can be seen behind Ross and Phoebe in JFK Airport when Phoebe claims they are on their honeymoon. When the camera cuts to Rachel at Newark Airport as she's looking for her boarding pass, those same people are there too. We are on to you bandana lady.

7. The one with the magic doodle - season 4, episode 20

Ross comes to Chandler and Joey's apartment to ask what the guys are doing in four weeks. The Magna Doodle behind Ross at first says "poop", then is blank. The camera cuts to Joey checking his diary and back to Ross. The Magna Doodle now reads "get out". Spooky....

8.  The one with the wrong arm - season 5, episode 23

Joey tells Chandler about his identical hand twin and plans for getting rich. When he sees Chandler's reaction, Joey points accusingly at Chandler with his left arm. In the next shot, he can be seen pointing with his right arm.

9. The one with the foam finger - season 1, episode 4

Ross can be seen wearing a foam finger at the hockey game, which later disappears. Although he may have just put it down, in fairness.

10.  The one with the Rachel stand-in - season 9, episode 15

Finally, Rachel is wearing a paisley top when she tells Joey that his agent called to say he got the audition for the Broadway play. In the next shot, Jennifer Aniston's stand-in can be seen standing beside Joey wearing a different top.