In the London flashback, Monica is wearing a different dress.
In the original episode where they are leaving for London, Chander's condom box is red. In the flashback, when Joey comes back to get the condoms from Chandler, the box is blue.
The London hotel room set has a number of changes to the original episode (s4 ep24) that it was featured in. The bed headboards are much taller, the pictures on the wall by the door are completely different, the 3 flower pictures on the wall above the beds are different, the lamp on the nightstand has a different color base and shade and there's a hotel card missing, replayed by an object with a Union Jack on it.
In a flashback to Chandler and Monica kissing in London, Chandler says "Well, I have kissed over four women!" Yet in previous seasons, we had seen him date Aurora, Janice, Kathy and other women, as well as mention the fact that he first touched a girl's breast at age 19.
On the close-up shot of Joey's "fried stuff with cheese" dinner, his fork is already on his plate with a chicken nugget on it. But when Joey sits down, he picks up the fork and stabs the nugget again.
When, Ross comes to Rachel's apartment to tell her that he's got a bone to pick with her about Ben pranking him, you can clearly see the black chair behind Rachel rocking back and forth with nobody sitting in it during a few shots then all the sudden it stops rocking. Correction - We can clearly see Rachel sitting on the chair at the start of the scene and rocking herself in it, when she gets up the chair still rocks back and forth by itself because it is a rocking chair, and during the scene the chair rocks slower and slower as the momentum dies out. By the end of the scene the chair just stops rocking as any chair that rocks would do by then.
Rachel says to Ben, "so, no brothers or sisters hunh?" while they are sitting on the couch. The camera angle changes to Ben and you can see Rachel mouthing the same words, but no audio is heard. Occurs at approximately 3:30.
In the flashback, it's revealed that Joey leaves Chandler one condom. But in "The One After Ross Says Rachel," which takes place the morning after Chandler and Monica sleep together, Chandler says they did it seven times.
During the London flashback, Joey walks into his and Chandler's hotel room and Monica dives under the covers. In this shot there is a microphone clearly visible in the top center of the screen.
When Joey walks in the room with Chandler and Monica to talk about his performing their wedding ceremony, one of the microphones is visible at the top left corner.