The One With The Worst Best Man Ever - S4-E22
Revealing mistake: In the scene when they confront the stripper at Chandler's office. Ross starts shouting about his grandmother's ring, and as Chandler says "way to be cool, man", you can see Joey ducking behind Ross, covering his mouth with laughter. A brief cut away, then back and he's standing with the others again looking serious. Widescreen version only.
The One With Chandler In A Box - S4-E8
Revealing mistake: When Chandler is in the box and Ross accidentally places his hand over the 'air-hole' you can see Ross feeling for the hole for a while before he finds it. (00:12:35)
The One Where They're Going To PARTY! - S4-E9
Revealing mistake: When Monica and Phoebe are in Central Perk, Phoebe is looking through the list of ideas that Monica has come up with for the use of Phoebe's van. If you look closely (most obvious just after Phoebe says "let's plan the wedding reception") you can see that every single page of the notebook Phoebe is holding has the same thing written on it, instead of lots of different ideas that Monica is meant to have had. (00:17:10)
The One Where They're Going To PARTY! - S4-E9
Revealing mistake: When Phoebe is looking at the front page of the "Chelsea Reporter," Monica says her restaurant review is on the back page. However, Phoebe doesn't turn to the back page, but merely flips the paper over, looking at the bottom of the front page (and finding the review there). (00:07:30)
The One With All The Haste - S4-E19
Revealing mistake: When they are using cards to bet on the Knicks tickets, watch Joey carefully as he goes to reshuffle the pack from behind the arm of the chair. It's obvious that there are two decks, and he is in fact switching them over. (00:13:40)
The One With Joey's Dirty Day - S4-E14
Revealing mistake: After Joey is caught in Charlton Heston's shower, he wraps a towel around his waist and sits on the arm of chair. You can see the line of the Speedo underwear under the towel. (00:15:55)
Revealing mistake: Before Phoebe agrees to carry Frank and Alice's' triplets, she already has a baby bump.
The One With The Ballroom Dancing - S4-E4
Revealing mistake: During the end bit, before we get to see Phoebe when she is being interviewed at Lara and Jeni's Massage, if you look closely at the name of the massage parlor, you'll see that, although the camera shakes and bumps, the words stay on the very same spot on the screen. Obviously the canopy was blank and the name was digitally added later.
The One With Phoebe's Uterus - S4-E11
Revealing mistake: Phoebe already has a visible baby bump before she even agrees to get pregnant.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.