In 'The One With The Cheap Wedding Dress', Monica trades the dress that she loves to another woman who wants the dress in exchange for booking the "Swing Kings". At the reception, the band is not the Swing Kings, but rather, "Jungle Swing". Both the dress and the Swing Kings were very important to Monica, and her control issues would never allow her to give up both.
Joey is revealed to have "freakishly tiny feet". For Chandler not to have noticed this before - through the many years of sharing an apartment, playing basketball and football together, taking camping trips together, visiting Joey in hospital (during his kidney stone ailment) and watching Joey in drama plays that included dance numbers - is totally inconceivable.
When Mona introduces herself to Ross, she mentions she works at Monica's restaurant. Yet in subsequent stories, she and Monica never say one word to each other, share a ride to/from work, or are even in the same frame together. This is odd, considering how much time and effort Monica invested in Ross' previous relationships with Julie and Emily.
Rachel, Phoebe and Monica are talking in the living room. The big box (present) to Monica that she started to unwrap before Rachel came in the door goes from being wrapped to unwrapped multiple times in that section.
In the original broadcast versions Ross appears to not interact directly with Ben at the children's table. However, in the extended DVD versions Ben has the line "Dad, can't you go back to your table?" after the second chair "fart" incident.
Joey says that if he could get in a Broadway show then he would've done it all: film, TV and theater, but he already was in a few plays in the earlier seasons.However, those were off-off-Broadway productions, and a far cry from a "real" Broadway show.
Monica refers to the reception as "the party" twice. In the previous season, she scolded Chandler for referring to the wedding as a party. Also, Rachel refers to it being a good party and Monica should have corrected her as well. However, Monica was originally upset about the wedding being called a party; she wouldn't mind if the reception were called a party.
When Monica has her back towards the camera while talking to Joey about wearing his tennis outfit, her wedding gown is being held together with a safety pin at the top and the buttons aren't through the button holes.
The goof item below may give away important plot points.
Rachel is four weeks pregnant at the time of Monica and Chandlers wedding, but only finds out the morning of the wedding. In an earlier episode, the wedding date is revealed to be May 15th, which would make Rachel's due date in mid-January, not summer as the show will later indicate.