The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Phoebe gives the housewarming gifts to Ross and also when Ross opens the door for the neighbor, his apartment number is "3B" and his neighbor's door is "3A," but in episode 14, "The One Where Everybody Finds Out," the door can be seen three times during the episode, and the number is "201" each time.
The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Ross tells Chandler and Rachel that he's running out to buy name tags for his party, in the next shot the water bottle beside Rachel does not have its cap on, but it cuts to Rachel's closeup and the cap is on. Other differences can be seen as well, such as Rachel's hair and an extra utensil.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: When Joey is counting the floors of Ross' building (the first time) to find out where Hot Girl lives, he is wearing a black top with a zip. When he rushes over to Ross' building he walks down the hall and knocks on what he believes to be Hot Girl's door. In these shots he is wearing a dark purple button up shirt. As soon as Ross answers the door, Joey is wearing the black top with a zip again. (00:03:15)
The One With Rachel's Inadvertent Kiss - S5-E17
Factual error: When Joey tries to find the Hot Girl, he goes up to Ross' apartment and there is a dead end. However, when he next goes there, he manages to run away in that direction. (00:03:20)
The One With Rachel's Inadvertent Kiss - S5-E17
Revealing mistake: When Ross is doing his "Watching TV" bit, there is a brief shot of the blank screen that he is sitting on the sofa pretending to watch. In the screen's reflection, it is clear that the sofa is empty. (00:03:55)
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: When Rachel returns from her first interview at Ralph Lauren she takes out a bottle of water out of the fridge. In the following shot her hands are empty and there are only Chandler's and Joey's bottles on the table. Rachel's bottle reappears on the table next to Chandler's and Joey's bottles (that are also standing closer together than before) for a moment just when Chandler starts saying "Well, you could have tried not kissing him" and then vanishes again. (00:06:20 - 00:07:15)
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: While Joey's looking for the "hot girl" the second time, after he knocks on the little girl's door and asks if her mother, sister, or babysitter are a hot girl, he runs off when she shouts for her father, and when Joey stops in another hallway full of doors the shadow of the moving boom mic can be seen on the door beside him.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Rachel gets back from her first interview, just as Chandler says, "Oh hey," we can see the boom mic at the top of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Joey is standing at Monica's window waving to Ross, the end of the set wall with movement on the soundstage can be seen at the left side of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: While the two couples are at the restaurant, Chandler has a beer and Gary, Monica, and Phoebe have margaritas, and the amount of liquid in Gary's margarita glass keeps changing, back and forth from less to more.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: After Monica tells Rachel that a guy from Ralph Lauren called and they want her for a second interview, when Rachel walks toward the TV and then when she stands in front of the TV, we can see a few tapes marks on the floor. One of those tape marks is also visible in one of Joey's closeups.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: Joey is watching Ross from Monica's window and goes searching for the hot girl. The exterior shot crossing the street between buildings is inconsistent with the interior shots, which show them looking at each other across an alley with no visible fire escapes. The exterior shot crosses a street to a building with street-facing fire escapes on every apartment.
The One Where Rachel Smokes - S5-E18
Continuity mistake: When Ben is playing with the building blocks, they change with each shot, and he holds different coloured blocks each time. (00:15:20)
The One Where Rachel Smokes - S5-E18
Continuity mistake: Joey's snowcone changes during the scene with Ross at the party. It goes from rounded to more flat to rounded, and from plain ice to being able to see the flavoring to ice again. (00:20:00)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: When Phoebe asks Rachel for the earrings she borrowed, Chandler is visible in the background, standing up with one leg on the table and talking to Joey. When the camera angle switches, Chandler is sitting down. (00:05:00)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: After Joey runs out of the apartment to go make the fake videotape, when it cuts to Rachel walking back into the apartment the table with the tablecloth is quite far away from the front door, but when Caitlin knocks and Ross rushes to go flirt with her that table is near the door. (00:13:35 - 00:15:15)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: In the scene where the gang is watching Joey's "substitution" video, you can easily see the reflection on the television that no one is sitting on the couch, but when the camera goes back to the gang, they're sitting down on the couch. (00:19:30)
The One Where Ross Can't Flirt - S5-E19
Other mistake: While everyone's watching Law and Order for Joey's big scene, Rachel, Phoebe, Monica, and Chandler are sitting on the couch and Ross is standing behind them, but in the TV's closeup the screen reflection reveals an empty couch.
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: When Ross first tries to flirt with the pizza girl he has money in his hand. When Chandler walks over he gives his own money for the pizza, she leaves, and Ross still has his money. In the next shot when he shuts the door, his hands are empty, then it is back in the next shot when Ross yells at Chandler.
The One With The Ride-Along - S5-E20
Continuity mistake: Toward the end of the episode, Rachel is back in Ross' apartment, trying to mend the ornament she broke when she was in there earlier. Ross comes in unexpectedly and she tosses the broken pieces into the lightshade, leaving the glue on the side table. In subsequent shots, the glue goes from lying on its side, to standing up, a number of times. (00:16:00)
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.