Kathy: So, what did you do today?
Chandler: Well, I had an appointment to get my hair cut...
Kathy: Oh, it looks great!
Chandler: ...and then it got cancelled.
The spare room (Rachel's) in Monica's apartment has a wall right opposite the door, which serves absolutely no purpose except to block the view into the room from the living room. The bedroom set isn't always built, just when needed, so having that wall makes life much easier. It's completely illogical though, as it's too thin to be a supporting wall, and where it is makes getting into the room a pain. See more...
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Continuity: At the beginning when Rachel falls and starts bleeding, you can see that the wound is somewhere up above her forehead because the blood is running from the wound up there. However, throughout the rest of the episode she has a bandaid going horizontally across her forehead, nowhere near where the apparent wound was.
Plot hole: Chandler has always loaned Joey money for various things (rent, acting classes, Christmas gifts, etc), so he shouldn't be surprised to learn that Joey doesn't have money for fancy dinners or birthday celebrations. Plus, Joey never seems to have a problem taking money/handouts from Chandler - why now? Also, Chandler explains that they're buying Ross a cake ($20), but why doesn't Monica (the chef and his sister) just make him one?
Factual error: In the one where Joey and Chandler lose baby Ben on a transit city bus, they later have to pick him up at the transit city headquarters. The man working there assumes that one of them is the father. There is no way that a city official would just give away a baby without checking for ID or a birth certificate or proof of fathership of the child.
Other: When Rachel leave her message on Ross' answering machine she mentioned the name Michael after 17 seconds and "I'm over you" after 25 seconds. When Ross listen to the tape he asks "Who is Michael ?" after 8 seconds. Rachel flips the phone out of his hand after 18 seconds so he couldn't heard her "I'm over you".