Corrected entry: Towards the start, when Chandler, Monica and Phoebe are listening through the walls, when they are listening to Rachel and Joey, and the camera slides "through" one room to another, you can see Phoebe's hand in front of the wall section that is in between the two rooms.
Corrected entry: In Rachel's hotel room, there is a door on the left of the wall that joins onto Monica and Chandler's room. Due to the layout of the rooms, the only place it could lead to is Monica and Chandler's room. However, when Ross is talking to Rachel and she's looking for Joey, she walks through this door and has a quick look around. Surely she would see Joey in there, or at least Monica, Chandler and Phoebe and ask if they knew where Joey was?
Corrected entry: When Phoebe tells Monica and Chandler that Rachel and Joey are kissing in the next room, at the time she says "It's Rachel and Joey.", you can see that the door leading to their room has been partially cut to achieve the effect of the camera going through the wall.
Corrected entry: When Ross walks into Monica and Chandler's hotel room, he is packed much earlier than both Monica and Chandler. However, in the episode after Monica and Chandler get married, Ross only signs out of the hotel a few minutes before check-out time. This is also implied by other characters who say that he does that all the time.