When Joey is reading Little Women, it looks like he's about halfway through the book. In the next scene, it looks like he's only read the first few pages.
Just before Monica enters her bedroom to find Richard waiting on her bed, the large window at the back of the apartment has its blinds halfway closed. But when the angle switches to inside her bedroom, the blinds on the window are suddenly all the way down.
Monica declines Richards offer to grab a meal, stating that it was too soon. Richard jokes that it wasn't, as he had lunch at 11. However, earlier in the video shop, Monica was fined for returning her video at "8:02".
When Phoebe gives her boyfriend Robert the stretchy pants, Phoebe is holding her jacket on her arms in front of her. Without showing her put it down it disappears then comes back in between camera views.
When Joey is trying to talk about The Shining in front of Rachel, he says "Remember when Danny sees those two blanks in the hallway" (referring to the twins) and "All blank and no blank make blank a blank blank" (referring to the famous line "All work and no play make Jack a dull boy". Those things do not happen in the book by Stephen King, and are exclusive to the 1980 movie adaptation.
Joey mentions that the main character Josephine "Jo" March from the book "Little Women" has a crush on her best friend Theodore "Laurie" Lawrence. Quite famously, Jo does not have a crush on Laurie; Laurie has a crush on Jo. When he declares himself she does not return his feelings, and the broken-hearted Laurie runs away to Europe and eventually develops feelings for Jo's sister Amy.
During the Opening scene in the Video Store, in the background Cobra (1986) can be seen in a section marked "Children", when Cobra is an R Rated Movie.
When Richard decides to surprise Monica, Rachel would have been able to let him in. However it seems unlikely that Monica would still have a key to Richard's apartment when she reciprocates, so it's a bit of a mystery how she got in without him knowing.
At the Central Perk while Robert is making a phone call, it's clear that the coffee cups Phoebe's holding are empty even when she just got them.
Rachel asks Joey for some ice and gets an ice tray from the freezer. She supposedly empties the ice into the blender she brought (off camera). You would expect to hear some noise but there is none. Also you can't see any ice in the blender even though Rachel apparently has emptied the tray because she refilled it.
Rachel goes to the freezer to remove the ice tray, sets it down on the counter, then goes back to the freezer and looks surprised as she takes out Joey's copy of The Shining. There was no reason for her to go back to the freezer a second time unless she was returning the ice tray, which she wasn't, as she hadn't yet emptied it. She didn't notice the book the first time, so she didn't make the second trip to retrieve that, either.
During the Opening scene in the Video Store, in the background Cobra (1986) can be seen in a section marked "Children", when Cobra is an R Rated Movie. To the left, you can see the original box of Disney's Beauty and the Beast (1991) under the "Classics" section. This is an error as the film was only six years old at the time of this episode and wasn't named as a Disney classic until well into the 2000s.
When Chandler and Ross are standing at the coffee counter discussing phoebes date "showing brain" - you can see him in the background, shorts clearly covering everything.
A mic bobs down from the top of the screen while Monica is putting club soda on a rag (before wiping the tomatoes off Richard's shirt).
When Joey is talking to Ross and Chandler in the coffee shop about The Shining he references the twins in the hallway and quotes, "All blank and no blank make a blank a blank blank." However, bother of these appear only in the movie and do not appear in the book.
Monica uses metaphors to imply she wants a future with Richard yet a few minutes later she contradicts her metaphors by saying getting over Richard was one of the hardest things she's ever done and can't do it again.