The One With The Cake - S10-E4
Revealing mistake: When Rachel and Ross are on the way back, if you look at Ross you can see a purple tinge to the side of his face. This is from projecting the background. (00:17:30)

Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One With The Cake - S10-E4
Revealing mistake: When Rachel and Ross are on the way back, if you look at Ross you can see a purple tinge to the side of his face. This is from projecting the background. (00:17:30)

The One With The Birth Mother - S10-E9
Visible crew/equipment: When Joey's telling Phoebe about having been at a nice restaurant with his date and how she took fries off his plate, in the shots facing Joey in the restaurant we can see the end of the set wall, with a 2x4 framed plywood board and a heavy duty extension cord hanging there.
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: The Magna-doodle on the door in the boys apartment has a different picture on it every episode. There is usually some tenuous connection between the picture and the plot of the episode.
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.