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.
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 Joey wakes up all covered in sand, his right "breast" has a big crack. But in the next shot, the crack has gone.
The sand at the beach house is wet in spots when they walk in and then in the next shot it is completely dry.
When Phoebe goes to the other Phoebe's house, it's raining, but when Phoebe gets into the house her hair and her clothes are dry.
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.
In the first season, Phoebe mentions a girlfriend named Abby who shaves her head bald. Bonnie says that it has been awhile since she's shaved her head, so, in actuality, this should be Abby. However Phoebe could easily have more than one friend who shaved their head.
Phoebe leaves the beach house to go to other Phoebe's place, but the taxi that she drives is still clearly visible in the background in several shots while she is supposed to be away. Phoebe Abbott's house could be within walking distance, so there would be no need to take the taxi.
When Rachel chases Ross with the nail polish brush, he pins her on the couch and wipes the brush on her forehead. But the brush is clean and dry and doesn't leave any polish behind, despite Rachel just having used it to paint Monica's nails.
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 Phoebe is crawling through the window and it closes on her waist, a crew members elbows are visible pushing the window down on her.
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.