When Ross goes to Phoebe's apartment, he comes bearing veggie hot dogs because he ruined her date with Mike. During their conversation, the camera cuts between them a couple times. Although Phoebe has paper towels in her hand, she does not appear to motion as if to tear one off, and neither of them seem to take a bit of the treat. but in between takes they suddenly switch from unscathed and on the counter surface to missing a bite and placed neatly on a paper towel.
When Chandler confesses that he was watching regular porn, Monica hugs him. After that, the right side of his collar goes up and down between shots.
When Chandler confesses to Monica that he was watching regular porn, at one point his arms are crossed on his chest. The camera changes angles and his arms are uncrossed and at his sides.
When Chandler turns on the TV to watch pay-per-view porn. it starts playing immediately without him having ordered it.
When Rachel is describing Ross's cavewoman fantasy to Monica, just after Joey enters the apartment, the camera pans to Monica sitting down at the table. As she is sitting, a woman can be seen in the shot with straight hair and a blue shirt in the seat where Rachel is sitting.
When Ross visits Mike for the first time to tell him that he was the one who made Phoebe cry, Ross says the line "Yeah, I-I said something stupid", but watch closely as Ross sits down on the couch--it can be seen that his lips never move.
When Joey suggests that Monica surprise Chandler she's excited about a "second honeymoon at the Tulsa Ramada." However, at the beginning of the scene at Chandler's hotel, the location is The Hyatt, which is actually a 20-minute drive from The Ramada.
Both Joey and Phoebe say they never had relationships that lasted longer than a month. However, Phoebe dated a cop, Gary, for longer than a month, and it is said in the earlier seasons that Joey had been with a girl called Angela DelVecchio for three years.
Phoebe says she's never lived with a man before, but in the early seasons, she lived with her boyfriend Gary--for two days.