When Rachel rushes out of her bedroom and announces she's going to London to tell Ross she loves him, the brown cardigan that's draped over her shoulders disappears and reappears between shots.
When Rachel rushes out of her bedroom and announces she's going to London to tell Ross she loves him, the edge of the apartment set next to Rachel's bedroom door is visible.
When Phoebe and Rachel are talking about the fact that Rachel is still in love with Ross, during one shot, Phoebe's necklace is caught around her right breast, in the next it's hanging between them, and in the next it's back where it started.
Phoebe is talking on the phone with Emily's mother and making a sandwich at the same time. While she is talking, Phoebe puts mayo on her sandwich but at the end of the conversation, there is no mayo.
Emily and Ross would not have been able to marry in the demolished hall. All wedding venues in England have to be properly licensed as such before being used, and the license would have been revoked when the hall was closed.
Emily tells Ross not to point his pants at her, but being British, she would have called the garment he was holding "trousers". However, seeing as how she had been dating Ross for several months and was often in NYC, she may have understandably adopted the American phraseology when speaking to Ross directly.
The wedding dress Emily wears is not quite the same as the one she had ordered from the bridal shop in The One with All the Wedding Dresses (1998). The dress she's wearing has a little lace at the top of the bodice, and she's also wearing a different veil. The dress did not come with the jacket Emily wears, but she could have purchased that separately.
In the very beginning of the scene where Phoebe and Rachel get the call from Joey, a crew member can be seen walking out of the shot on the left.
When Monica, Ross and Emily are walking towards the wedding venue discussing food a camera track is visible on the footpath to the right of the screen towards the end.
During the establishing shot of the London Marriott hotel, as the shot pans round a reflection of the camera can be seen in the hotel's name sign.
At one point, Joey turns on the TV and begins watching Cheers (1982). The theme song should be heard playing at a slightly higher tempo and a semitone higher as the UK Video signal, PAL, requires it to be sped up by 5%.
When Monica, Ross and Emily are on their way to the church Monica frowns on the fact that Ross and Emily decides to serve chicken on their wedding - meaning that something like salmon are more classy than chicken. However, she is serving chicken at her own wedding. In season 7, episode 7 (The One with Ross's Library Book (2000)) Janice comes to the restaurant where Monica works to complain about the chicken. Later when Monica tells Chandler that Janice invited herself to the wedding she exclaims, "Oh my god, she's not gonna like the chicken that night either is she?"