The One With Ross's Teeth - S6-E8
Character mistake: While on his date with Hillary, Ross says that he has a five year old son. Ben was born in 1995 and this episode is set in 1999, so Ben should be four, not five.
Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One With Ross's Teeth - S6-E8
Character mistake: While on his date with Hillary, Ross says that he has a five year old son. Ben was born in 1995 and this episode is set in 1999, so Ben should be four, not five.

The One Where Ross Dates a Student - S6-E18
Visible crew/equipment: While Phoebe and Rachel are with Monica in the guestroom, Phoebe decides that Rachel should be the one to get the nice room, and right before Monica tells Rachel that she's the first guest at "Hotel Monica" we can see the end of the bedroom set wall, with the kitchen and living room visible.
The One With Chandler In A Box - S4-E8
Monica: Fine, judge all you want to, but [to Ross] married a lesbian, [to Rachel] left a man at the altar, [to Phoebe] fell in love with a gay ice dancer, [to Joey] threw a girl's wooden leg in a fire, [to Chandler] LIVING IN A BOX!
Trivia: Matthew Perry's wit is so legendary that the scriptwriters have often incorporated his gags into the show.
The One With the Sharks - S9-E4
Question: What's the name of the "scary painting" that Joey saw in the apartment of the girl he thought he'd slept with before? The strange and creepy painting (black and white, with an amorphous body and a chair). I really, really want to know who is the artist. Not Gladys - that's Phoebe's painting that comes out of the frame.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.