When Monica is on the phone with Chandler and Joey they ask her what she is wearing and she says nothing but rubber gloves and they come bursting through the door seconds later, but the door to their apartment is closed. If they ran from their apartment that fast, they wouldn't have had time to close their door on their way to Monica and Rachel's apartment.
When Rachel is holding Ben in the coffeehouse, she pulls up his shirt as she takes him from Ross. His shirt then goes from halfway up his stomach to normal length between shots.
When Richard is playing foosball with Chandler and Joey, right before Monica comes over to tell him dinner is ready, the camera, while on Richard, pans too far to the right past the edge of the fake apartment "wall" and the hallway set behind them is visible.
After Joey says that he and Chandler will invite Richard to the Knicks game only if he takes his Jaguar, Chandler drops his head. In the next shot, he's looking up and slightly to his right, mouthing something. In the next shot, he's looking at Joey.
When Ben is saying "bye", a couple of times his lips clearly aren't moving.
When Phoebe describes the films shes shocked by, she mentions a soccer team eating each other in the Andes. Its actually a rugby union team and not a soccer team, The Old Christians from Montevideo Uruguay as shown in the film Alive.
Chandler explains to Richard their context of "dad" by saying his not familiar with their "young persons vernacular". This phrase contradicts his explanation.