When Chandler and Monica are in the tub and Monica says that Chandler looks cute in bubbles, her hair is dry. However when Joey knocks on the door, her hair is suddenly wet.
In one of the kissing scenes, Joey refers to Monica as "mon frere" to mean "mon ami" while approaching her to attempt a kiss, but later (in Season 8) when Joey is trying to confess his feelings for Rachel to Chandler, he doesn't understand the term when Chandler refers to him as "mon frere."
When Phoebe's waters break, as she's just about to go through the door Chandler steps around the "water" and is a distance behind Joey and Rachel, but in the next shot he is standing between them.
After Rachel and Monica have spoken in the hallway and Monica is trying to get back in, she is shown from the hallway banging on the door with her bare left hand, while the laundry basket is in her right hand. In the next shot, as Ross opens the door, she is now holding the laundry basket in her left arm, and banging on the door with her right hand, which now has a sock on it. There was no time between shots for her to have switched hands and added the sock.
When Rachel locks Monica out of the apartment, Monica starts banging on the door with her left hand. In the next scene, she's banging with her right hand, and she's wearing a sock on her hand. Also, the laundry basket switches from her right side to her left.
When the gang suggests a picnic in Central Park, Phoebe is less than impressed. "What's so great about Central Park, it's right by my house," she says (or something to that effect). Phoebe lives at 5 Morton St in the West Village, which is quite a hike from Central Park.
In the first scene when Ross is telling everyone he loves Emily he has no ring on his finger, towards the end of that scene when leaves to order the 72 roses you can see the ring on his finger.
Monica tells Rachael, 'I just ran into Dave and he told me you blew him off' when talking in the hallway. However, she had less than five seconds in which to have this meeting and conversation.
When Joey offers to break down Monica's door if she'll give him "a little sugar," a crew member is visible hiding behind Joey's door.
Rachel pops into the hallway to get rid of her date Dave, and when she opens the door to the apartment to go back in, a crew member is visible through the doorway.
The gang talk about the things they did in London (making Phoebe feel left out), however Rachel was only in London for the wedding, so she couldn't have been there.
Monica shows a picture to Pheobe saying that is everybody in Tower of London. But actually the picture is from Westminster Abbey.