The boys insist on leaving the porn on 24/7, yet it is clearly off when Emily is in the apartment, on the phone with Ross. It is back on in the next scene.
At Central Perk when Phoebe is telling Alice and Frank Jr. about having triplets the legs of the girl over Phoebe's shoulder are crossed right over left, then left over right, then right over left with each cut.
When Rachel walks over to the guys to turn off the TV while they're eating, Chandler says "no one touches the air around the TV." Then in the following shots/counter-shots, Rachel's position switches from hands on her hips to arms crossed in front of her.
The flight that Emily gets on to go back to London is Virgin Atlantic flight VS009. This flight is actually a LHR-JFK flight number so should really be VS010 which is the correct flight number for the flight JFK-LHR.
A number of time, Chandler starts reacting early to Joey's lines before Joey has even communicated them. As Joey says in another episode, an important part of acting is reacting.
When Emily is on her way back to London, her flight number is VS009 (visible on the board by the gate). In reality, flight VS009 is Virgin Atlantic's flight at the opposite direction - from London to New York.