Pheobe's hairstyle changes in between shots
The hummus stain on Phoebe's dress changes in size between shots.
When Chandler comes back from getting dressed, it's Joey's turn to go change which is when he takes the "essence" (the cushions) of the chair along with him. Then, Joey gets back and he accuses Chandler of wearing his underwear. Chandler claims he stole the underwear only because Joey took his essence, but Chandler already put on Joey's underwear before Joey stole the cushions, so there's no way Chandler wore the underwear as revenge for taking the cushions as that has not happened yet.
At the end, when Ross and Rachel are about to leave the apartment, the place where Rachel stands changes between shots. Just before when she says "Oh, and by the way (...)" we see the door behind here, but on the very next shot we see her on the other side of the door (they are only half-open) and a little closer to the center of the room.
When Chandler comes back in after getting dressed, he's not wearing a tie. After the "chair" incident, he's standing near the TV and is fiddling with a bow-tie. In the next scene, he's sitting on the arm of the chair and is wearing the bow-tie.
Monica fixates on a message left by Richard on her answering machine, not knowing if the message is old or new. However, when Monica checks the machine the "voice" says there are "two new messages". The gang then hears messages from Phoebe and Ross. It isn't until after those two messages are played do they hear the one from Richard. The machine only identified two "new" messages: those from Ross and Phoebe. The message from Richard is obviously old.