The One With Ross's Inappropriate Song - S9-E7
Other mistake: When Joey and Chandler are watching Richard's sex tape the screen is off right before Monica comes in, and yet she sees the video playing (you can still hear sex noises with the screen off). (00:14:40)
The One With Ross's Inappropriate Song - S9-E7
Other mistake: When Chandler finds out that the video marked 'Monica' is just a football game he says, 'This may be the first time I have enjoyed football.' Although in 'TOW The Football' (season 3) he is quite happily watching the Thanksgiving Day game with Ross and Joey.