When Ross takes the pie out of the guys hood, there is no paper under it. When the guy turns around, there is a paper under the slice of pie that changes position several times.
When Joey uses the bathroom in the previous episode, he comes back (from the right side) and he mentions to Chandler that there were mirrors on each side of him. Later (this episode), after Joey moves into the apartment and is showing the others around, the bathroom is off to the left and there aren't any mirrors around the toilet - just a phone.
When Joey is in the coffee house talking to Monica and Phoebe about moving back in with Chandler, you can see the microphone.
When Monica is pounding on the bathroom door to get Ross out, the closeup is of her right hand and her watch is on her right wrist. Monica always wears her watch on her left wrist, and in the next scene you can see her watch is on her left wrist. The likely cause of this is the close-up image was reversed.
In The One with the Baby on the Bus (1995), Phoebe chastises the professional singer (Chrissie Hynde) for saying the phrase "from the top," meaning to start over at the beginning of the song, but Phoebe doesn't chastise the record producer when she says the same phrase.