At the end of this episode, Ross opens the door of Rachel's room, says, "Hi," and then closes it. However, in the next episode (The One With The Jellyfish), this scene is shown again and changed: Ross opens the door of Rachel's room, says, "Hi," and doesn't close it - he talks with Rachel and Bonnie with the door open.
When Phoebe breaks into Phoebe Abbott's house, after older Phoebe confronts her with the coat hanger and they're talking, the bedroom door that she came out of is closed. While they are talking, the door suddenly swings open as if someone opened it.
When Phoebe goes to visit her mum for the first time, before she takes the photo off the fridge the cookie jar has its lid on. After she has taken the photo off the fridge, the lid has come off the cookie jar though there is no way Phoebe senior could have taken it off.
During the last shot, when Ross opens Rachel's bedroom door, the doorway is right next to the far wall of Rachel's room. At the beginning of the next episode when we see Ross inside the room, there is enough space between the far wall and the doorway for a dresser.
When Bonnie first arrives at the beach house, when she enters the door look at the top of the screen above the left door and you will see a microphone dip into the shot for a second before the camera changes.
During the scene when Phoebe breaks into the other Phoebe's house and gets caught, the older Phoebe was telling the younger one that the man in the photo is not Frank Buffay, but is Chuck Mangioni. Right after that, as Phoebe 2 walks towards the camera, you can see that the door behind her moves open, and there is a visible shadow of someone who walks by that doorway.
When the gang all see the beach house for the first time, it's covered in a layer of sand, which is supposedly left over from a recent flood. If there really had been a flood, there would have been mold on the walls and debris or junk from the outside, and the sand wouldn't have been nice and clean and dry; it would have been moldy. And there would've been a pattern, likely on the walls, of where the water rose up to and where it receded. Not only that, but how did it get in? All of the doors and windows are intact.