The One With Rachel's Date - S8-E5
Revealing mistake: In the scene in the coffee house, Phoebe is talking to Monica who is drinking a cup of coffee. Towards the end of the scene, the camera is on Phoebe who is smiling and laughing at Monica. In the right-hand corner is a side view of 'Monica'; however, it is not Courteney Cox in the scene but a completely different actress. [Widescreen version only, although the "wrong" nose is visible in the original]. (00:06:20)
Revealing mistake: When Ellen and the rest are on the ski lift, it becomes apparent that they are being filmed in front of a green screen because as the lift chair in front of them sways back and forth it disappears into it on the left side. (00:02:05)