In The One with the Chicken Pox (1996) when Monica is dating Richard, she specifies that she must sleep on her side of the bed, but Chandler is on that side when they're arguing that Ross stole the joke and that Phoebe picked Rachel.
When Monica asks Joey if he's working as a waiter in Central Perk, Phoebe is sitting on the couch on the right side of Rachel. Shots before and after do not show Phoebe sitting beside Rachel.
(at around 4 mins) When Rachel explains the woman on the horse in Playboy and says, "Where are they? Where are they?", her hands are in front and above the magazine. In next shot, the hands are different.
Gunther says he's going out for an hour to have his hair dyed, but when he gets back it looks exactly the same.
When Chandler and Ross are giving their explanations of how they each came up with the joke, Ross says, "...remember, evolution- monkey into Man...". As a paleontologist, Ross would know that the Theory of Evolution does not propose humans evolved from apes but that apes and humans have a common ancestor.
When Joey is in the coffee shop right after Gunther leaves, a customer goes to sit down (JD Lewis). Joey says "that seat is taken" as the man goes to get up you can see a camera in the upper left corner of the screen at 23.03 minutes.