The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: At the start of the first exterior shot of the London Marriott Hotel, when the camera pans to the left and reaches the awning the reflection of the camera/crane can be seen, and we also see it turn and lower.
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: After Ross and Monica talk about Emily's dream of the perfect wedding and perfect guy (and Monica's pillowcase), when it cuts to Phoebe and Rachel at the apartment we can see a crewmember (wearing a blue shirt) at the left side of the screen, before Phoebe mentions that she thought of a good flaw. (DVD).
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Joey, wearing the Union Jack hat, walks into the hotel room while Chandler's sitting on the bed, on the widescreen version we can see the end of the set wall at the right side of the screen. On the DVD, just as Joey walks across the room studio lighting can be seen at the top left side of the screen.
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: Rachel is cutting flowers and she and Phoebe are discussing if she loves Ross. Phoebe's necklace switches sides every time the shot goes back to her. (00:07:35)
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: When Monica, Ross and Emily are going to see the church, Emily's hands move between shots with no time to do so. (00:05:30)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Rachel gets back from shopping, all the stage lights can be seen in the tv. (00:11:50)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: In the scene where Phoebe and Rachel are discussing whether she loves Ross, the stage lights are visible in the TV. (00:07:25)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Rachel exits her bedroom and says she's going to London, you can see tape markings on the floor by her bedroom door and the end of the set. Furthermore, you can see a chair which is likely part of the Central Perk set. [Widescreen version]. (00:20:25)
The One With Ross's Wedding, part 1 - S4-E23
Other mistake: When Phoebe tells Rachel that she thinks she can help her get over Ross, she starts by saying that she needs some photos of Ross. Rachel immediately starts walking towards the kitchen, walking right past the red box of photos on the table beside Phoebe (which she will pick up on her way back and then hand to Phoebe), before Phoebe even mentions the chocolate (and then glass of water) she also needs. It's obvious that Rachel is headed for the kitchen before she knows she needs to go there. (00:12:00)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Rachel opens the door to the apartment while she's holding flowers and sees Phoebe trying to get up from the couch (before the wedgie comment), the open edge of the set wall can be seen at the top left corner of the screen.
The One With Ross's Wedding, part 1 - S4-E23
Plot hole: The wedding is threatened because Ross, Emily, and Monica discover that the location has been torn down a few days early. However, when one plans an event, especially a wedding, one puts down a deposit to hold the location, signs a contract, and has all of that in writing. The church would not have allowed the tear-down to happen early as they'd be in breach of contract for accepting money and then allowing the church to be torn down before the wedding. And if they had broken the contract, Monica would have been screaming about it and demanding a refund.
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.