When Rachel goes to Ross' apartment to tell him about her and Joey, Rachel puts down her handbag on the coffee table twice.
In the previous episode The one in Barbados, when Rachel enters Chandler and Monica's room to tell them it's sunny and they can go the beach, we can see there's no door on the right side on the bed. But in this episode, we can see a door has appeared and we then learn it's the bathroom door (when Chandler comes out of it), yet Chandler and Monica didn't change room.
At the part where Ross tells Chandler, Monica, and Phoebe to tell Joey that he's looking for him, the hallway is shown. If you watch closely, in the darkened left corner of the hallway, there is a toilet. It most likely doubled as a bathroom. Response: It is a planter.
Based on where the doors, windows and bathroom doors are in each of their rooms, there's no way Monica and Chandler's room could be between Rachel's and Ross's rooms.
When Joey and Rachel are kissing, Jennifer Anniston's tan line from her wedding ring is clearly visible.
When Monica, Chandler, and Phoebe are listening through Ross's wall (before the theme song), the camera scrolls past the wall, showing it is just one wall dividing the rooms. In later shots, however (like when the door's open), you can see that they are divided by a hallway as well.
In season nine finale Rachel had flip flops that matched her dress, in the premiere of season ten the first shot of flip flops are a hot pink. They would not match Rachel's dress.
When the group is on the plane and Chandler is walking to his seat you can clearly see a camera man in the top left corner.
In this episode, when Rachel and Joey are in Joey's apartment at the end, the camera hole in Joey's wall next to the door is visible twice.
In the room where Monica and Chandler are staying, you can sometimes see the top of the back set wall. Right before the scene where they board the plane, you can see the top of the wall, as well as a boom microphone and set lights.
Charlie mentions seeing a documentary about hotel bedspreads and infra red light. In fact, the technology that reveals hidden biological agents on surfaces uses ultraviolet light, not infra red.
Ross comes into Chandler and Monica's room to ask if he was ready to go and Chandler notes he's still packing, right before Phoebe and Monica show up from the salon. Presumably, this means Ross is packed up and ready to depart. However, in an earlier season where the group is staying in a hotel for Chandler and Monica's wedding, they say that Ross NEVER checks out of a hotel a minute before he has to. He should be the last one to be ready.