The Porsche's license plate is YSO 791 when it's parked outside Central Perk; but later, when we see Ross driving it, the license plate has changed to JGL 334.
Chandler turns off the lamp on the table to avoid attention when his father starts working the audience. After the awkwardness, his father goes over to another table and the lamp is on.
When Monica and Chandler are talking in the living room, the bedroom door is open and closed between shots.
When Joey says that pantyhose go all the way up to here, his hands are near his hips, but when he says, "I should go take these off, shouldn't I," there is a close up that shows his hands are in his lap.
When Rachel climbs into Monica's Porsche right after Ross has driven it, the driver's seat is in the perfect position for her without her having to adjust it. It should be much farther back to accommodate his longer legs.
Monica won't let Rachel drive the Porsche because she's a bad driver. Interestingly, she lets Phoebe drive it all the time even though she has been shown to be a terrible driver too. However, it's possible Monica doesn't known how bad of a driver Phoebe is since we've never seen Monica as one of Phoebe's passengers.
When Joey is talking to Phoebe's boyfriend Jake, Joey says, "Maybe you and I should get to know each other a little bit." The camera pans over to Jake, who says, "Sure, I'd like that," however his mouth doesn't move at all.
The shadow of the boom mic and its operator falls across the hood of the Porsche as Ross tries to stop Rachel from driving it.
In season 4, Rachel tells Gunther that her birthday is May 5th. Therefore, she is a Taurus - not an Aquarius.