Emily was unable to find the wedding dress she wanted in her size in England, so she had Ross ask Monica to pick it up in New York. The dress had a veil and no jacket, but at the wedding, Emily is wearing a jacket and no veil.
Right before the wedding when Felicity comes out before she asks Joey to "Talk New York to me again", she closes the door behind her. In the very next shot when they start making out, the same door is wide open.
The dress Emily wears at the wedding is not the same dress Monica wears in the apartment,the top part of the dress Monica wears is plain with thin straps,whereas Emilys dress has extra material on the top edge,also Monica is wearing a veil where Emily doesn't.
Joey says in his best man speech at the rehearsal that he met Ross "In this coffeehouse back home". Even though it was originally a pool hall/bar, he may have forgotten because he was too emotional from being homesick.
Even though there are many legal and logistical issues to take into account when getting married in England, the filmmakers excluded them to simplify the story.
When Ross and Emily's parents meet for the first time, Judy Geller makes the joke that this may be the only wedding they get to throw, referring to Monica never getting married. However, Ross was already married once before to Carol.
However, assuming Ross' wedding to Carol followed the more common tradition where the bride's family pays for the wedding, it's likely that Ross' parents didn't really play any part in throwing that wedding.
While the Gellers are fighting with Emily's parents, Ross threatens them by saying "no grandchildren." However, Ross already had his son, Ben, three years earlier, and therefore the Gellers already have a grandchild. What Ross meant was that he and Emily would have no grandchildren during their marriage.
When Rachel is rushing into the apartment to retrieve her passport she is dragging her luggage with her. Assuming she came by cab, even though she could have left her luggage with the cab, it's possible she had simply forgotten because she was more focused on telling Ross her feelings for him.
Hugh Laurie is credited as "The Gentleman on the Phone" instead of "The Gentleman on the Plane" - an obvious result of incorrect interpretation of someone's handwriting. This error is corrected on the DVD.
A number of odd liberties were taken with the wedding processional. An extra groomsman is seen walking bridesmaid Felicity down the aisle, after which, he disappears. This was done so that Joey could walk Emily's stepmother down the aisle and end up talking to Phoebe via the stepmother's phone.
When Chandler comes out of the bathroom and Joey tells him about Rachel coming to London he is wearing socks despite just getting out of the shower. The water can be heard running when Joey comes in, Chandler is wearing just a robe and Chandler's hair is wet.
When Ross's father gets in a fight with Emily's father and Ross is trying to calm them down, Ross's voice does not match his lips for a few words.
While Steven is negotiating the bill with Ross, Andrea interjects and visibly mouths "Will you say something, Steven - please!" but there is no audio or her saying "please."