On the Etch-a-Sketch on the wall in Joey and Chandler's kitchen, the words "get out!" are written when Rachel is getting a drink from the fridge. It changes to "PooP" when Rachel is sitting at the breakfast bar talking to the guys, but after Chandler dances and Ross enters, the writing on the toy changes back to "get out!"
The background painting in the coffee house changes from the the blue animal in the first scene at the coffee house to the coffee cup painting in the scene with Rachel and Joshua.
When Phoebe shows up at Monica's apartment wearing a wedding dress, she's wearing a veil with a tiara on top of her head, and her hair is down. A little bit later, when they're pretending to catch the bouquet, Phoebe's hair is up, and the veil is on the back of her head with no tiara.
[Around 8min] when Chandler is covering his eyes after rolling Joey over, you can see the wall behind him in the background is fake and has no upper portion or ceiling.
When Chandler is waking up Joey at the sleep clinic he starts with his left hand but changes to his right hand when the camera angle changes.
Why would Phoebe buy a maternity wedding dress, put it on before she got to Monica's apartment, then knock frantically as if she was embarrassed to be seen in it. Why didn't she bring it to her apartment then put it on instead of running around town in it.